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Document and Entity Information (USD  $)	6 Months Ended
Jun. 30, 2011	Jul. 22, 2011	Jun. 30, 2010
Document and Entity Information [Abstract]
Entity Registrant Name	BIOGEN IDEC INC.
Entity Central Index Key	0000875045
Document Type	10-Q
Document Period End Date	Jun 30, 2011
Amendment Flag	false
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	Q2
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 25,908,506,479.17
Entity Common Stock, Shares Outstanding		242,545,771

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Condensed Consolidated Statements of Income (Unaudited) (USD  $) In Thousands, except Per Share data	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Revenues:
Product	$ 956,703	$ 859,235	$ 1,863,805	$ 1,683,455
Unconsolidated joint business	216,458	306,371	472,583	561,300
Other	35,486	47,096	75,602	76,807
Total revenues	1,208,647	1,212,702	2,411,990	2,321,562
Cost and expenses:
Cost of sales, excluding amortization of acquired intangible assets	100,503	106,985	203,616	204,040
Research and development	285,644	331,675	579,277	638,705
Selling, general and administrative	266,301	262,322	510,819	510,987
Collaboration profit sharing	88,050	62,692	162,844	126,249
Amortization of acquired intangible assets	55,136	53,148	108,352	102,037
Acquired in-process research and development				39,976
Restructuring charge			16,587
Fair value adjustment of contingent consideration	2,200		3,400
Total cost and expenses	797,834	816,822	1,584,895	1,621,994
Income from operations	410,813	395,880	827,095	699,568
Other income (expense), net	(11,728)	1,012	(1,777)	(7,373)
Income before income tax expense	399,085	396,892	825,318	692,195
Income tax expense	95,036	102,243	212,504	177,553
Net income	304,049	294,649	612,814	514,642
Net income attributable to noncontrolling interests, net of tax	16,015	1,211	30,450	3,762
Net income attributable to Biogen Idec Inc.	$ 288,034	$ 293,438	$ 582,364	$ 510,880
Net income per share:
Basic earnings per share attributable to Biogen Idec Inc.	$ 1.19	$ 1.13	$ 2.4	$ 1.92
Diluted earnings per share attributable to Biogen Idec Inc.	$ 1.18	$ 1.12	$ 2.38	$ 1.91
Weighted-average shares used in calculating:
Basic earnings per share attributable to Biogen Idec Inc.	242,375	259,938	241,932	265,018
Diluted earnings per share attributable to Biogen Idec Inc.	244,966	261,658	244,899	267,272

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Condensed Consolidated Balance Sheets (Unaudited) (USD  $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Current assets:
Cash and cash equivalents	$ 697,526	$ 759,598
Marketable securities	629,218	448,146
Accounts receivable, net	666,960	605,329
Due from unconsolidated joint business	184,871	222,459
Inventory	308,254	289,066
Other current assets	201,794	215,822
Total current assets	2,688,623	2,540,420
Marketable securities	1,183,559	743,101
Property, plant and equipment, net	1,712,869	1,641,634
Intangible assets, net	1,678,867	1,772,826
Goodwill	1,146,314	1,146,314
Investments and other assets	211,747	248,198
Total assets	8,621,979	8,092,493
Current liabilities:
Current portion of notes payable, line of credit and other financing arrangements	131,981	137,153
Taxes payable	10,330	84,517
Accounts payable	170,746	162,529
Accrued expenses and other	641,273	665,923
Total current liabilities	954,330	1,050,122
Notes payable and line of credit	1,062,986	1,066,379
Long-term deferred tax liability	196,784	200,950
Other long-term liabilities	351,685	325,599
Total liabilities	2,565,785	2,643,050
Commitments and contingencies (Notes 2, 11, 16, 18 and 20)
Biogen Idec Inc. shareholders' equity
Preferred stock, par value  $0.001 per share	0	0
Common stock, par value  $0.0005 per share	127	124
Additional paid-in capital	4,224,865	3,895,103
Accumulated other comprehensive income (loss)	25,713	(21,610)
Retained earnings	2,454,848	1,872,481
Treasury stock, at cost	(728,503)	(349,592)
Total Biogen Idec Inc. shareholders' equity	5,977,050	5,396,506
Noncontrolling interests	79,144	52,937
Total equity	6,056,194	5,449,443
Total liabilities and equity	$ 8,621,979	$ 8,092,493

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Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD  $)	Jun. 30, 2011	Dec. 31, 2010
Biogen Idec Inc. shareholders' equity
Preferred stock, par value	$ 0.001	$ 0.001
Common stock, par value	$ 0.0005	$ 0.0005

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Condensed Consolidated Statements of Cash Flows (Unaudited) (USD  $) In Thousands	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010
Cash flows from operating activities:
Net income	$ 612,814	$ 514,642
Adjustments to reconcile net income to net cash flows from operating activities:
Depreciation and amortization of property, plant and equipment and intangible assets	182,845	169,961
Acquired in-process research and development		39,976
Share-based compensation	57,399	95,370
Fair value adjustment of contingent consideration	3,400
Excess tax benefit from share-based compensation	(37,827)	(5,598)
Deferred income taxes	48,626	(30,317)
Write-down of inventory to net realizable value	7,296	5,654
Impairment of marketable securities, investments and other assets	6,137	17,231
Non-cash interest (income) expense, foreign exchange remeasurement loss (gain), net and other	9,748	6,858
Realized gain on sale of marketable securities and strategic investments	(15,539)	(11,300)
Changes in operating assets and liabilities, net:
Accounts receivable	(54,909)	(22,955)
Due from unconsolidated joint business	37,588	(56,354)
Inventory	(24,708)	21,447
Other assets	(30,354)	3,637
Accrued expenses and other current liabilities	(67,488)	(23,732)
Other liabilities and taxes payable	(51,784)	40,856
Net cash flows provided by operating activities	683,244	765,376
Cash flows from investing activities:
Proceeds from sales and maturities of marketable securities	1,169,836	2,002,543
Purchases of marketable securities	(1,778,568)	(941,268)
Acquisitions		(39,976)
Purchases of property, plant and equipment	(86,229)	(85,260)
Purchases of intangible assets	(14,505)
Purchases of other investments	(3,954)	(2,338)
Proceeds from the sale of strategic investments	39,835
Net cash flows (used in) provided by investing activities	(673,585)	933,701
Cash flows from financing activities:
Purchase of treasury stock	(386,575)	(1,609,334)
Proceeds from the issuance of stock for share-based compensation arrangements	285,883	63,193
Excess tax benefit from share-based compensation	37,827	5,598
Change in cash overdraft	4,485	2,912
Net contributions (to) from noncontrolling interests	(9,930)	2,187
Repayments of borrowings	(7,248)	(14,142)
Repayments on financing arrangement for the sale of the San Diego facility	(2,367)
Net cash flows used in financing activities	(77,925)	(1,549,586)
Net increase in cash and cash equivalents	(68,266)	149,491
Effect of exchange rate changes on cash and cash equivalents	6,194	(10,418)
Cash and cash equivalents, beginning of the period	759,598	581,889
Cash and cash equivalents, end of the period	$ 697,526	$ 720,962

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Business	6 Months Ended
Jun. 30, 2011
Business and Collaborations [Abstract]
Business	1.   Business   Overview   Biogen Idec is a global biotechnology company focused on discovering, developing, manufacturing and marketing products for the treatment of serious diseases with a focus on neurological disorders. We currently have four marketed products: AVONEX, RITUXAN, TYSABRI, and FUMADERM. Our marketed products are used for the treatment of multiple sclerosis (MS), non-Hodgkin’s lymphoma (NHL), rheumatoid arthritis (RA), Crohn’s disease, chronic lymphocytic leukemia (CLL), and psoriasis.   Basis of Presentation   In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our consolidated financial statements and the accompanying notes included in our Annual Report on Form 10-K for the year ended December 31, 2010 (2010 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2010 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three and six months ended June 30, 2011 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.   Consolidation   Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities in which we are the primary beneficiary. For consolidated entities in which we own less than a 100% interest, we record net income (loss) attributable to noncontrolling interests in our consolidated statement of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. All material intercompany balances and transactions have been eliminated in consolidation.   In determining whether we are the primary beneficiary of an entity, we apply a qualitative approach, that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative and joint venture relationships and determine whether we consolidate companies or entities with which we have collaborative or other arrangements. Determination about whether an enterprise should consolidate a variable interest entity is required to be evaluated continuously as changes to existing relationships or future transactions may result in us consolidating or deconsolidating our partner(s) to collaborations and other arrangements.   Use of Estimates   The preparation of our condensed consolidated financial statements in accordance with U.S. GAAP requires management to make estimates, judgments, and assumptions that may affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosure of contingent assets and liabilities. On an on-going basis, we evaluate our estimates and judgments and methodologies, including those related to revenue recognition and related allowances, our collaborative relationships, clinical trial expenses, the consolidation of variable interest entities, the valuation of contingent consideration resulting from a business combination, the valuation of acquired intangible assets including in-process research and development, inventory, impairment and amortization of long-lived assets including intangible assets, impairments of goodwill, share-based compensation, income taxes including the valuation allowance for deferred tax assets, valuation of investments, derivatives and hedging activities, contingencies, litigation, and restructuring charges. We base our estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results may differ from these estimates under different assumptions or conditions.   Subsequent Events   We did not have any material recognizable subsequent events. However, we did have the following non-recognizable subsequent events:     •  On July 20, 2011, the European Commission (EC) granted a conditional marketing authorisation for FAMPYRA (prolonged release fampridine) in the E.U., which triggered a  $25.0 million milestone payment payable to Acorda Therapeutics, Inc. (Acorda). FAMPYRA is an oral compound indicated as a treatment to improve walking ability in people with MS.     •  On July 14, 2011, we executed leases for two office buildings to be built in Cambridge, Massachusetts. These buildings, totaling approximately 500,000 square feet, will serve as the future location of our corporate headquarters and commercial operations. The buildings will also provide additional general and administrative and research and development office space. For a more detailed description of these transactions, please read Note 11, Property, Plant and Equipment to these condensed consolidated financial statements.

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Acquisitions	6 Months Ended
Jun. 30, 2011
Acquisitions [Abstract]
Acquisitions	2.   Acquisitions   Acquisition of Panima Pharmaceuticals AG   On December 17, 2010, we acquired 100% of the stock of Panima Pharmaceuticals AG (Panima), an affiliate of Neurimmune AG. The purchase price was comprised of a  $32.5 million cash payment plus up to  $395.0 million in contingent consideration payable upon the achievement of development milestones. Panima is a business involved in the discovery of antibodies designed to treat neurological disorders.   Upon acquisition, we recorded a liability of  $81.2 million representing the acquisition date fair value of the contingent consideration. Subsequent changes in the fair value of this obligation are recognized as adjustments to contingent consideration within our consolidated statements of income. As of June 30, 2011, the fair value of the total contingent consideration obligation within our condensed consolidated balance sheet was  $84.6 million, of which  $4.9 million was reflected as a component of accrued expenses and other, and  $79.7 million was reflected as a component of other long-term liabilities. We recorded contingent consideration expense of  $2.2 million and  $3.4 million for the three and six months ended June 30, 2011, respectively, reflecting the change in the fair value of this obligation. For additional information related to this transaction, please read Note 2, Acquisitions to our consolidated financial statements included within our 2010 Form 10-K.   Acquisition of Biogen Idec Hemophilia Inc.   In connection with our acquisition of Biogen Idec Hemophilia Inc. (BIH), formerly Syntonix Pharmaceuticals, Inc. (Syntonix), in January 2007, we agreed to make additional milestone payments associated with the development of long-lasting recombinant Factor IX, a product for the treatment of hemophilia B. In January 2010, we initiated patient enrollment in a registrational trial of Factor IX, which triggered an approximately  $40.0 million milestone payment to the former shareholders of Syntonix. We recorded this payment as a charge to acquired in-process research and development within our condensed consolidated statement of income for the six months ended June 30, 2010, in accordance with the accounting standards applicable to business combinations when we acquired BIH.

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Restructuring	6 Months Ended
Jun. 30, 2011
Restructuring [Abstract]
Restructuring	3.   Restructuring   In November 2010, we announced a number of strategic, operational, and organizational initiatives designed to provide a framework for the future growth of our business and realign our overall structure to become a more efficient and cost effective organization. As part of this initiative:     •  We have out-licensed, terminated or are in the process of discontinuing certain research and development programs, including those in oncology and cardiovascular medicine, that are no longer a strategic fit for us.     •  We have completed a 13% reduction in workforce spanning our sales, research and development, and administrative functions.     •  As of June 30, 2011, we have vacated the San Diego, California facility and consolidated certain of our Massachusetts facilities. For a more detailed description of transactions affecting our facilities, please read Note 11, Property, Plant and Equipment to these condensed consolidated financial statements.   Based upon our most recent estimates, we expect to incur total restructuring charges of approximately  $100.0 million associated with the implementation of these initiatives, which we expect will be substantially incurred and paid by the end of 2011. Costs associated with our workforce reduction primarily relate to employee severance and benefits. Facility consolidation costs are primarily comprised of charges associated with closing these facilities, related lease obligations and additional depreciation recognized when the expected useful lives of certain assets have been shortened due to the consolidation and closing of related facilities and the discontinuation of certain research and development programs. We incurred  $16.6 million of these charges in the six months ended June 30, 2011 and  $75.2 million of these charges in the fourth quarter of 2010.   For the three and six months ended June 30, 2011, we recognized restructuring charges of  $1.5 million and  $6.0 million, respectively, in relation to the consolidation of our facilities inclusive of amounts related to additional depreciation. For the six months ended June 30, 2011, we recognized net restructuring charges of  $10.5 million in relation to our workforce reduction initiatives. Restructuring charges related to workforce reduction for the three months ended June 30, 2011 reflect  $2.7 million of expense offset by net adjustments of  $4.4 million, which primarily resulted from revisions to our previous estimates for health and welfare benefit costs for terminated employees.   The following table summarizes the activity of our restructuring liability:                                 Workforce     Facility         (In millions)   Reduction     Consolidation     Total     Restructuring reserve as of December 31, 2010    $ 60.6      $ 5.8      $ 66.4   Expense     12.1       2.4       14.5   (Payments) receipts, net     (73.8 )     (2.0 )     (75.8 ) Adjustments to previous estimates, net     (1.6 )     —       (1.6 ) Other adjustments     8.6       (3.2 )     5.4                             Restructuring reserve as of June 30, 2011    $ 5.9      $ 3.0      $ 8.9

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Revenue Recognition	6 Months Ended
Jun. 30, 2011
Revenue Recognition [Abstract]
Revenue Recognition	4.   Revenue Recognition   We recognize revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; our price to the customer is fixed or determinable; and collectibility is reasonably assured.   Product Revenues   Revenues from product sales are recognized when title and risk of loss have passed to the customer, which is typically upon delivery. However, sales of TYSABRI in the U.S. are recognized on the “sell-through” model, that is, upon shipment of the product by Elan Pharma International, Ltd. (Elan), an affiliate of Elan Corporation, plc., to its third party distributor rather than upon shipment to Elan. Product revenues are recorded net of applicable reserves for discounts and allowances.   Reserves for Discounts and Allowances   We establish reserves for trade term discounts, wholesaler incentives, Medicaid rebates, Veterans Administration (VA) and Public Health Service (PHS) discounts, managed care rebates, product returns and other governmental rebates or applicable allowances. Reserves established for these discounts and allowances are classified as reductions of accounts receivable (if the amount is payable to our direct customer) or a liability (if the amount is payable to a party other than our customer). In addition, we distribute no-charge product to qualifying patients under our patient assistance and patient replacement goods program. This program is administered through one of our distribution partners, which ships product to qualifying patients from its own inventory received from us. Gross revenue and the related reserves are not recorded on product shipped under this program and cost of sales is recorded when the product is shipped.   Product revenue reserves are categorized as follows: discounts, contractual adjustments and returns. An analysis of the amount of, and change in, reserves is summarized as follows:                                               Contractual               (In millions)   Discounts     Adjustments     Returns     Total     Balance, as of December 31, 2010    $ 13.9      $ 107.0      $ 21.1      $ 142.0   Current provisions relating to sales in current year     47.2       174.6       6.8       228.6   Adjustments relating to prior years     —       (8.4 )     —       (8.4 ) Payments/returns relating to sales in current year     (33.3 )     (101.4 )     (0.3 )     (135.0 ) Payments/returns relating to sales in prior years     (13.0 )     (58.5 )     (4.9 )     (76.4 )                                   Balance, as of June 30, 2011    $ 14.8      $ 113.3      $ 22.7      $ 150.8                                       Our product revenue reserves are based on estimates of the amounts earned or to be claimed on the related sales. These estimates take into consideration our historical experience, current contractual and statutory requirements, specific known market events and trends and forecasted customer buying patterns. Actual amounts may ultimately differ from our estimates. If actual results vary, it will result in an adjustment to these estimates, which could have an effect on earnings in the period of adjustment.   During the six months ended June 30, 2011, we reduced our reserves for contractual adjustments by  $8.4 million, which was primarily due to a revision of our previous estimates associated with the impact of healthcare reform.     The total reserves above, included in our condensed consolidated balance sheets, are summarized as follows:                         As of     As of       June 30,     December 31,   (In millions)   2011     2010     Reduction of accounts receivable    $ 38.4      $ 36.7   Current liability     112.4       105.3                     Total reserves    $ 150.8      $ 142.0                       Revenues from Unconsolidated Joint Business   We collaborate with Genentech on the development and commercialization of RITUXAN. Revenues from unconsolidated joint business consist of (1) our share of pre-tax co-promotion profits in the U.S.; (2) reimbursement of our selling and development expense in the U.S.; and (3) revenue on sales of RITUXAN in the rest of world, which consists of our share of pretax co-promotion profits in Canada and royalty revenue on sales of RITUXAN outside the U.S. and Canada by F. Hoffmann-La Roche Ltd. (Roche) and its sublicensees. Pre-tax co-promotion profits are calculated and paid to us by Genentech in the U.S. and by Roche in Canada. Pre-tax co-promotion profits consist of U.S. and Canadian sales of RITUXAN to third-party customers net of discounts and allowances less the cost to manufacture RITUXAN, third-party royalty expenses, distribution, selling and marketing, and development expenses incurred by Genentech, Roche and us. We record our share of the pretax co-promotion profits in Canada and royalty revenues on sales of RITUXAN outside the U.S. on a cash basis. Additionally, our share of the pretax co-promotion profits in the U.S. includes estimates supplied by Genentech.   Royalty Revenues   We receive royalty revenues on sales by our licensees of other products covered under patents that we own. We do not have future performance obligations under these license arrangements. We record these revenues based on estimates of the sales that occurred during the relevant period. The relevant period estimates of sales are based on interim data provided by licensees and analysis of historical royalties that have been paid to us, adjusted for any changes in facts and circumstances, as appropriate. We maintain regular communication with our licensees in order to assess the reasonableness of our estimates. Differences between actual royalty revenues and estimated royalty revenues are adjusted in the period in which they become known, typically the following quarter. Historically, adjustments have not been material when compared to actual amounts paid by licensees. If we are ever unable to accurately estimate revenue, then we record revenues on a cash basis.

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Accounts Receivable	6 Months Ended
Jun. 30, 2011
Accounts Receivable [Abstract]
Accounts Receivable	5.   Accounts Receivable   Our accounts receivable primarily arise from product sales in the U.S. and Europe and primarily represent amounts due from our wholesale distributors, large pharmaceutical companies, public hospitals and other government entities. The majority of our accounts receivable have standard payment terms which are generally between 30 and 90 days. We monitor the financial performance and credit worthiness of our large customers so that we can properly assess and respond to changes in their credit profile. We provide reserves against trade receivables for estimated losses that may result from a customer’s inability to pay. Amounts determined to be uncollectible are charged or written-off against the reserve. To date, such losses have not exceeded management’s estimates.   Concentrations of credit risk with respect to receivables, which are typically unsecured, are limited due to the wide variety of customers and markets using our products, as well as their dispersion across many different geographic areas. We monitor economic conditions, including volatility associated with international economies, and related impacts on the relevant financial markets and our business, especially in light of sovereign credit issues. The credit and economic conditions within Italy, Spain, Portugal and Greece, among other members of the European Union, have deteriorated. These conditions have increased, and may continue to increase, the average length of time that it takes to collect on our accounts receivable outstanding in these countries.   Our net accounts receivable balances from product sales in these countries are summarized as follows:                                 As of June 30, 2011     Balance Included   Balance Included         within Accounts   within Investments     (In millions)   Receivable, Net   and Other Assets   Total   Italy    $ 116.5      $ 11.3      $ 127.8   Spain    $ 81.7      $ 36.7      $ 118.4   Portugal    $ 23.9      $ 7.4      $ 31.3   Greece    $ 10.7      $ —      $ 10.7                                   As of December 31, 2010     Balance Included   Balance Included         within Accounts   within Investments     (In millions)   Receivable, Net   and Other Assets   Total   Italy    $ 103.2      $ 14.8      $ 118.0   Spain    $ 70.8      $ 29.8      $ 100.6   Portugal    $ 17.8      $ 5.5      $ 23.3   Greece    $ 3.9      $ —      $ 3.9     Of the amounts included within the tables above, approximately  $53.5 million and  $45.0 million were outstanding for more than one year as of June 30, 2011 and December 31, 2010, respectively. Amounts included as a component of investments and other assets within our condensed consolidated balance sheets represent amounts that are expected to be collected beyond one year.   In May 2011, European Union finance ministers approved a three-year EUR78 billion rescue package for Portugal. Under the terms of the package, Portugal is required to correct its excessive deficit by 2013 and improve the efficiency and effectiveness of its health care system, including through austerity measures aimed at reducing healthcare costs. These measures include plans to standardize control procedures to reduce outstanding balances payable to drug suppliers.   Our concentrations of credit risk related to our accounts receivable from product sales in Greece to date have been limited as our receivables within this market are due from our distributor. These receivables remain current and substantially in compliance with their contractual due dates.

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Inventory	6 Months Ended
Jun. 30, 2011
Inventory [Abstract]
Inventory	6.   Inventory   The components of inventory are summarized as follows:                         As of     As of       June 30,     December 31,   (In millions)   2011     2010     Raw materials    $ 59.8      $ 59.0   Work in process     165.6       142.2   Finished goods     82.9       87.9                     Total inventory    $ 308.3      $ 289.1

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Intangible Assets and Goodwill	6 Months Ended
Jun. 30, 2011
Intangible Assets and Goodwill [Abstract]
Intangible Assets and Goodwill	7.   Intangible Assets and Goodwill   Intangible Assets   Intangible assets, net of accumulated amortization, impairment charges and adjustments, are summarized as follows:                                                                       As of June 30, 2011     As of December 31, 2010       Estimated           Accumulated                 Accumulated         (In millions)   Life     Cost     Amortization     Net     Cost     Amortization     Net     Out-licensed patents     12 years      $ 578.0      $ (370.9 )    $ 207.1      $ 578.0      $ (350.2 )    $ 227.8   Core developed technology     15-23 years       3,005.3       (1,723.4 )     1,281.9       3,005.3       (1,636.9 )     1,368.4   In process research and development     Up to 15 years upon commercialization       110.9       —       110.9       110.9       —       110.9   Trademarks and tradenames     Indefinite       64.0       —       64.0       64.0       —       64.0   In-licensed patents     Up to 14 years       17.5       (2.5 )     15.0       3.0       (1.3 )     1.7   Assembled workforce     4 years       2.1       (2.1 )     —       2.1       (2.1 )     —   Distribution rights     2 years       12.7       (12.7 )     —       12.7       (12.7 )     —                                                             Total intangible assets            $ 3,790.5      $ (2,111.6 )    $ 1,678.9      $ 3,776.0      $ (2,003.2 )    $ 1,772.8                                                               Our most significant intangible asset is the core technology related to our AVONEX product. The net book value of this asset as of June 30, 2011 was  $1,268.8 million.   In the first quarter of 2011, we entered into a license agreement granting us exclusive patent rights for the diagnostic and therapeutic application of recombinant virus-like particles, known as VP1 proteins. These VP1 proteins are used to detect antibodies of the JC virus (JCV) in serum or blood. Under the terms of this agreement, we expect to make payments totaling approximately  $47.1 million through 2016. These payments include upfront and milestone payments as well as the greater of an annual maintenance fee or usage-based royalty payment. As of June 30, 2011, we recognized an intangible asset in the amount of  $14.5 million, reflecting the total of upfront payments made and other time-based milestone payments expected to be made. We will further capitalize additional payments due under this arrangement as an intangible asset as they become payable. We will amortize the intangible asset resulting from these payments utilizing an economic consumption amortization model with the amount of amortization determined by the ratio of actual JCV assay tests performed in the current period to the total number of JCV assay tests expected to be performed through 2016.   For the three and six months ended June 30, 2011, amortization for acquired intangible assets totaled  $55.1 million and  $108.4 million, respectively, as compared to  $53.1 million and  $102.0 million, respectively, in the prior year comparative periods. Amortization for acquired intangible assets is expected to be in the range of approximately  $180.0 million to  $220.0 million annually through 2015.   Other than the amounts recorded in connection with the license agreement described above, total intangible assets was unchanged as of June 30, 2011 compared to December 31, 2010, excluding the impact of amortization.   Goodwill   Our goodwill balance remained unchanged as of June 30, 2011 compared to December 31, 2010. As of June 30, 2011, we had no accumulated impairment losses.

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Fair Value Measurements	6 Months Ended
Jun. 30, 2011
Fair Value Measurements [Abstract]
Fair Value Measurements	8.   Fair Value Measurements   A majority of our financial assets and liabilities have been classified as Level 2. Our financial assets and liabilities (which include our cash equivalents, derivative contracts, marketable debt securities, and plan assets for deferred compensation) have been initially valued at the transaction price and subsequently valued, at the end of each reporting period, typically utilizing third party pricing services or other market observable data. The pricing services utilize industry standard valuation models, including both income and market based approaches and observable market inputs to determine value. These observable market inputs include reportable trades, benchmark yields, credit spreads, broker/dealer quotes, bids, offers, current spot rates and other industry and economic events. We validate the prices provided by our third party pricing services by reviewing their pricing methods and matrices, obtaining market values from other pricing sources, analyzing pricing data in certain instances and confirming that the relevant markets are active. After completing our validation procedures, we did not adjust or override any fair value measurements provided by our pricing services as of June 30, 2011 and December 31, 2010.   Our strategic investments in publicly traded equity securities are classified as Level 1 assets as their fair values are readily determinable and based on quoted market prices.   We also maintain venture capital investments classified as Level 3 whose fair value is initially measured at transaction prices and subsequently valued using the pricing of recent financing or by reviewing the underlying economic fundamentals and liquidation value of the companies. These investments are the only investments for which we used Level 3 inputs to determine the fair value and represented approximately 0.2% and 0.3% of our total assets as of June 30, 2011 and December 31, 2010, respectively. These investments include investments in certain biotechnology oriented venture capital funds which primarily invest in small privately-owned, venture-backed biotechnology companies. The fair value of our investments in these venture capital funds has been estimated using the net asset value of the fund. The investments cannot be redeemed within the funds. Distributions from each fund will be received as the underlying investments of the fund are liquidated. The funds and therefore a majority of the underlying assets of the funds will not be liquidated in the near future. The underlying assets in these funds are initially measured at transaction prices and subsequently valued using the pricing of recent financings or by reviewing the underlying economic fundamentals and liquidation value of the companies that the funds invest in. We apply judgments and estimates when we validate the prices provided by third parties. While we believe the valuation methodologies are appropriate, the use of valuation methodologies is highly judgmental and changes in methodologies can have a material impact on our results of operations. Gains and losses (realized and unrealized) included in earnings for the period are reported in other income (expense), net.   In addition, in the fourth quarter of 2010, we recognized an in-process research and development asset and recorded a contingent consideration obligation related to our acquisition of Panima. Upon acquisition, we recorded a liability of  $81.2 million representing the acquisition date fair value of the contingent consideration. Subsequent changes in the fair value of this obligation are recognized as adjustments to contingent consideration within our consolidated statement of income. We determined the fair value of the contingent consideration obligation based upon probability-weighted assumptions related to the achievement of certain milestone events and thus the likelihood of us making payments. We revalue the acquisition-related contingent consideration obligation on a recurring basis each reporting period. This fair value measurement is based on inputs not observable in the market and therefore represents a Level 3 measurement.   The fair value of our Level 3 contingent consideration obligation as of June 30, 2011 and December 31, 2010, was  $84.6 million and  $81.2 million, respectively. These valuations were determined based upon net cash outflow projections of  $395.0 million, discounted using a rate of 5.7% and 6.1%, respectively, which is the cost of debt financing for market participants. The change in fair value of this obligation, of  $2.2 million and  $3.4 million for the three and six months ended June 30, 2011, respectively, was primarily due to changes in the discount rate and in the expected timing related to the achievement of certain developmental milestones and was recognized as a fair value adjustment of contingent consideration within our condensed consolidated statements of income for the three and six months ended June 30, 2011.   There were no transfers between fair value measurement levels during the six months ended June 30, 2011.   The tables below present information about our financial assets and liabilities that are measured at fair value on a recurring basis as of June 30, 2011 and December 31, 2010, and indicate the fair value hierarchy of the valuation techniques we utilized to determine such fair value:                                                           Significant       Balance as of     Quoted Prices in     Significant Other     Unobservable       June 30,     Active Markets     Observable Inputs     Inputs   (In millions)   2011     (Level 1)     (Level 2)     (Level 3)     Assets:                                 Cash equivalents    $ 440.6      $ —      $ 440.6      $ —   Marketable debt securities:                                 Corporate debt securities     475.1       —       475.1       —   Government securities     1,095.4       —       1,095.4       —   Mortgage and other asset backed securities     242.3       —       242.3       —   Strategic investments     2.0       2.0       —       —   Venture capital investments     20.6       —       —       20.6   Derivative contracts     0.5       —       0.5       —   Plan assets for deferred compensation     15.2       —       15.2       —                                     Total    $ 2,291.7      $ 2.0      $ 2,269.1      $ 20.6                                     Liabilities:                                 Derivative contracts    $ 24.9      $ —      $ 24.9      $ —   Acquisition-related contingent consideration     84.6       —       —       84.6                                     Total    $ 109.5      $ —      $ 24.9      $ 84.6                                                                                               Significant       Balance as of     Quoted Prices in     Significant Other     Unobservable       December 31,     Active Markets     Observable Inputs     Inputs   (In millions)   2010     (Level 1)     (Level 2)     (Level 3)     Assets:                                 Cash equivalents    $ 651.8      $ —      $ 651.8      $ —   Marketable debt securities:                                 Corporate debt securities     313.0       —       313.0       —   Government securities     785.3       —       785.3       —   Mortgage and other asset backed securities     92.9       —       92.9       —   Strategic investments     44.8       44.8       —       —   Venture capital investments     20.8       —       —       20.8   Derivative contracts     1.3       —       1.3       —   Plan assets for deferred compensation     13.0       —       13.0       —                                     Total    $ 1,922.9      $ 44.8      $ 1,857.3      $ 20.8                                     Liabilities:                                 Derivative contracts    $ 12.2      $ —      $ 12.2      $ —   Acquisition-related contingent consideration     81.2       —       —       81.2                                     Total    $ 93.4      $ —      $ 12.2      $ 81.2                                       The following table provides a roll forward of the fair value of our venture capital investments, which are all Level 3 assets:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Beginning balance    $ 20.5      $ 20.8      $ 20.8      $ 21.9   Unrealized gains included in earnings     0.1       —       0.7       —   Unrealized losses included in earnings     (0.3 )     (0.1 )     (1.3 )     (1.6 ) Purchases     0.3       (0.3 )     0.4       0.1   Issuances     —       —       —       —   Settlements     —       —       —       —                                     Ending balance    $ 20.6      $ 20.4      $ 20.6      $ 20.4                                         The fair and carrying values of our debt instruments, which are all Level 2 liabilities, are summarized as follows:                                         As of June 30, 2011     As of December 31, 2010       Fair     Carrying     Fair     Carrying   (In millions)   Value     Value     Value     Value     Credit line from Dompé    $ 4.3      $ 4.3      $ 8.1      $ 8.0   Note payable to Fumedica     24.4       21.7       24.2       22.0   6.0% Senior Notes due 2013     482.9       449.8       485.5       449.8   6.875% Senior Notes due 2018     639.4       595.1       618.0       597.9                                     Total    $ 1,151.0      $ 1,070.9      $ 1,135.8      $ 1,077.7                                       The fair values of the credit line from Dompé Farmaceutici SpA and our note payable to Fumedica were estimated using market observable inputs, including current interest and foreign currency exchange rates. The fair value of our Senior Notes was determined through market, observable, and corroborated sources.

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Financial Instruments	6 Months Ended
Jun. 30, 2011
Financial Instruments [Abstract]
Financial Instruments	9.   Financial Instruments   Marketable Securities, including Strategic Investments   The following tables summarize our marketable securities and strategic investments:                                               Gross     Gross             Fair     Unrealized     Unrealized     Amortized   As of June 30, 2011 (In millions)   Value     Gains     Losses     Cost     Available-for-sale                                 Corporate debt securities                                 Current    $ 122.3      $ 0.2      $ —      $ 122.1   Non-current     352.8       1.3       (0.2 )     351.7   Government securities                                 Current     504.7       0.3       —       504.4   Non-current     590.7       0.8       (0.1 )     590.0   Mortgage and other asset backed securities                                 Current     2.2       —       —       2.2   Non-current     240.1       0.5       (0.6 )     240.2                                     Total available-for-sale securities    $ 1,812.8      $ 3.1      $ (0.9 )    $ 1,810.6                                     Other Investments                                 Strategic investments, non-current    $ 2.0      $ 0.5      $ —      $ 1.5                                                                                   Gross     Gross             Fair     Unrealized     Unrealized     Amortized   As of December 31, 2010 (In millions)   Value     Gains     Losses     Cost     Available-for-sale                                 Corporate debt securities                                 Current    $ 93.2      $ 0.1      $ —      $ 93.1   Non-current     219.8       2.1       (0.5 )     218.2   Government securities                                 Current     352.8       0.2       —       352.6   Non-current     432.5       0.6       (0.6 )     432.5   Mortgage and other asset backed securities                                 Current     2.1       —       —       2.1   Non-current     90.8       0.5       (0.2 )     90.5                                     Total available-for-sale securities    $ 1,191.2      $ 3.5      $ (1.3 )    $ 1,189.0                                     Other Investments                                 Strategic investments, non-current    $ 44.8      $ 17.5      $ —      $ 27.3                                       In the tables above, as of June 30, 2011 and December 31, 2010, government securities included  $247.0 million and  $163.5 million, respectively, of Federal Deposit Insurance Corporation (FDIC) guaranteed senior notes issued by financial institutions under the Temporary Liquidity Guarantee Program.   The following table summarizes our financial assets with original maturities of less than 90 days included within cash and cash equivalents on the accompanying condensed consolidated balance sheet:                         As of     As of       June 30,     December 31,   (In millions)   2011     2010     Commercial paper    $ 8.0      $ 4.0   Repurchase agreements     80.9       26.0   Short-term debt securities     351.7       621.8                     Total    $ 440.6      $ 651.8                       The carrying values of our commercial paper, including accrued interest, repurchase agreements, and our short-term debt securities approximate fair value.   Summary of Contractual Maturities: Available-for-Sale Securities   The estimated fair value and amortized cost of securities, excluding strategic investments, available-for-sale by contractual maturity are summarized as follows:                                         As of June 30, 2011     As of December 31, 2010       Estimated     Amortized     Estimated     Amortized   (In millions)   Fair Value     Cost     Fair Value     Cost     Due in one year or less    $ 629.2      $ 628.7      $ 448.1      $ 447.8   Due after one year through five years     1,040.9       1,039.0       664.1       662.4   Due after five years     142.7       142.9       79.0       78.8                                     Total    $ 1,812.8      $ 1,810.6      $ 1,191.2      $ 1,189.0                                         The average maturity of our marketable securities as of June 30, 2011 and December 31, 2010 was 13 months and 11 months, respectively.   Proceeds from Marketable Securities   The proceeds from maturities and sales of marketable securities, excluding strategic investments and resulting realized gains and losses, are generally reinvested, and are summarized as follows:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Proceeds from maturities and sales    $ 381.8      $ 973.2      $ 1,169.8      $ 2,002.5   Realized gains    $ 0.7      $ 7.4      $ 3.1      $ 13.1   Realized losses    $ (0.5 )    $ 1.1      $ (1.3 )    $ 1.8     In the first quarter of 2011, we also recognized within other income (expense), a net gain of  $13.8 million on the sale of stock from our strategic investment portfolio.   Impairments   We conduct periodic reviews to identify and evaluate each investment that has an unrealized loss in accordance with the meaning of other-than-temporary impairment and its application to certain investments.   For the three and six months ended June 30, 2011, we recognized  $5.5 million and  $6.8 million, respectively, in charges for the impairment of our investments in venture capital funds and investments in privately-held companies. No impairments were recognized in relation to our publicly-held strategic investments.   For the three and six months ended June 30, 2010, we recognized  $1.2 million and  $17.0 million, respectively, in charges for the impairment of our publicly-held strategic investments, investments in venture capital funds and investments in privately-held companies.

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Derivative Instruments	6 Months Ended
Jun. 30, 2011
Derivative Instruments [Abstract]
Derivative Instruments	10.   Derivative Instruments   Foreign Currency Forward Contracts   Due to the global nature of our operations, portions of our revenues are earned in currencies other than the U.S. dollar. The value of revenues measured in U.S. dollars is subject to changes in currency exchange rates. In order to mitigate these changes we use foreign currency forward contracts to lock in exchange rates associated with a portion of our forecasted international revenues.   Foreign currency forward contracts in effect as of June 30, 2011 and December 31, 2010 had durations of 1 to 12 months. These contracts have been designated as cash flow hedges and accordingly, to the extent effective, any unrealized gains or losses on these foreign currency forward contracts are reported in accumulated other comprehensive income (loss). Realized gains and losses for the effective portion of such contracts are recognized in revenue when the sale of product in the currency being hedged is recognized. To the extent ineffective, hedge transaction gains and losses are reported in other income (expense), net.   The notional value of foreign currency forward contracts that were entered into to hedge forecasted revenue is summarized as follows:                         Notional Amount       As of     As of       June 30,     December 31,   Foreign Currency (In millions)   2011     2010     Euro    $ 537.1      $ 460.3   Canadian dollar     11.4       24.0   Swedish krona     4.8       9.9                     Total foreign currency forward contracts    $ 553.3      $ 494.2                       The portion of the fair value of these foreign currency forward contracts that was included in accumulated other comprehensive income (loss) within total equity reflected net losses of  $23.3 million and  $11.0 million as of June 30, 2011 and December 31, 2010, respectively. We expect all contracts to be settled over the next 12 months and any amounts in accumulated other comprehensive income (loss) to be reported as an adjustment to revenue. We consider the impact of our and our counterparties’ credit risk on the fair value of the contracts as well as the ability of each party to execute its obligations under the contract. As of June 30, 2011 and December 31, 2010, credit risk did not materially change the fair value of our foreign currency forward contracts.   In relation to our foreign currency forward contracts, we recognized in other income (expense), net losses of  $1.2 million and  $0.5 million due to hedge ineffectiveness for the three and six months ended June 30, 2011, respectively, as compared to net losses of  $0.6 million and  $0.5 million, respectively, in the prior year comparable periods.   In addition, we recognized in product revenue net losses of  $18.5 million and  $26.8 million for the settlement of certain effective cash flow hedge instruments for the three and six months ended June 30, 2011, respectively, as compared to net gains of  $19.7 million and  $19.9 million, respectively, in the prior year comparable periods. These settlements were recorded in the same period as the related forecasted revenue.   Summary of Derivatives Designated as Hedging Instruments   The following table summarizes the fair value and presentation in our condensed consolidated balance sheets for derivatives designated as hedging instruments:                         Fair Value         As of June 30, (In millions)   Balance Sheet Location   2011   Foreign Currency Contracts             Asset derivatives   Other current assets    $ —   Liability derivatives   Accrued expenses and other    $ 23.8                           Fair Value         As of December 31, (In millions)   Balance Sheet Location   2010   Foreign Currency Contracts             Asset derivatives   Other current assets    $ —   Liability derivatives   Accrued expenses and other    $ 11.0       The following table summarizes the effect of derivatives designated as hedging instruments within our condensed consolidated statements of income:                                         Amount       Amount             Recognized in       Reclassified from             Accumulated       Accumulated             Other       Other             Comprehensive       Comprehensive             Income (Loss)   Income   Income (Loss)   Income   Amount of     on Derivative   Statement   into Income   Statement   Gain/(Loss)     Gain/(Loss)   Location   Gain/(Loss)   Location   Recorded (In millions)   (Effective Portion)   (Effective Portion)   (Effective Portion)   (Ineffective Portion)   (Ineffective Portion)   For the Three Months Ended                                 June 30, 2011:                       Other income         Foreign currency contracts    $ (23.3 )   Revenue    $ (18.5 )   (expense)    $ (1.2 ) June 30, 2010:                       Other income         Foreign currency contracts    $ 62.1     Revenue    $ 19.7     (expense)    $ (0.6 )                                   For the Six Months Ended                                 June 30, 2011:                       Other income         Foreign currency contracts    $ (23.3 )   Revenue    $ (26.8 )   (expense)    $ (0.5 ) June 30, 2010:                       Other income         Foreign currency contracts    $ 62.1     Revenue    $ 19.9     (expense)    $ (0.5 )   Other Derivatives   We also enter into other foreign currency forward contracts, usually with one month durations, to mitigate the foreign currency risk related to certain balance sheet positions. We have not elected hedge accounting for these transactions.   The aggregate notional amount of our outstanding foreign currency contracts was  $243.2 million as of June 30, 2011. The fair value of these contracts was a net liability of  $0.7 million. A net gain of  $0.6 million and a net loss of  $4.3 million related to these contracts were recognized as a component of other income (expense), net, for the three and six months ended June 30, 2011, respectively, as compared to net gains of  $7.9 million and  $13.1 million in the prior year comparative periods.

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Property, Plant and Equipment	6 Months Ended
Jun. 30, 2011
Property, Plant and Equipment [Abstract]
Property, Plant and Equipment	11.   Property, Plant and Equipment   Property, plant and equipment are recorded at historical cost, net of accumulated depreciation. Accumulated depreciation on property, plant and equipment was  $771.1 million and  $767.2 million as of June 30, 2011 and December 31, 2010, respectively.   San Diego Facility   On October 1, 2010, we sold the San Diego facility for cash proceeds, net of transaction costs, of approximately  $127.0 million. As part of this transaction, we agreed to lease back the San Diego facility for a period of 15 months. We are accounting for this transaction as a financing arrangement as we have determined that the transaction does not qualify as a sale due to our continuing involvement under the leaseback terms. Accordingly, we recorded an obligation for the proceeds received in October and the facility assets remain classified as held for use and the carrying value of the facility remains reflected as a component of property, plant and equipment, net within our condensed consolidated balance sheets as of June 30, 2011 and December 31, 2010. Our remaining obligation, which is reflected as a component of current portion of notes payable, line of credit and other financing arrangements within our condensed consolidated balance sheets, was  $124.5 million and  $125.9 million as of June 30, 2011 and December 31, 2010, respectively. We have not recognized a loss or impairment charge related to the San Diego facility.   In the first quarter of 2011, we terminated our 15 month lease of the San Diego facility effective August 31, 2011 and will have no continuing involvement or remaining obligation after that date. Once the lease arrangement has concluded we will account for the San Diego facility as a sale of property.   Hillerød, Denmark Facility   As of June 30, 2011 and December 31, 2010, the construction in progress balance related to the construction of our large-scale biologic manufacturing facility in Hillerød, Denmark totaled  $494.2 million and  $440.2 million, respectively. This facility is intended to manufacture large molecule products. In connection with our construction of this facility, we capitalized interest costs totaling approximately  $7.2 million and  $14.4 million for the three and six months ended June 30, 2011, respectively.   Based on our current manufacturing utilization strategy, we plan to begin manufacturing product in 2012, upon completion of the facility’s process validation activities.   New Cambridge Leases   In July 2011, we executed leases for two office buildings to be built in Cambridge, Massachusetts. We expect construction to begin in late 2011, with a planned occupancy during the second half of 2013. These buildings, totaling approximately 500,000 square feet, will serve as the future location of our corporate headquarters and commercial operations. These buildings will also provide additional general and administrative and research and development office space. The leases both have 15 year terms and we have options to extend the term of each lease for two additional five-year terms. Future minimum rental commitments under these leases will total approximately  $340.0 million over the initial 15 year lease terms. In addition to rent, the leases require us to pay additional amounts for taxes, insurance, maintenance and other operating expenses.

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Equity	6 Months Ended
Jun. 30, 2011
Equity and Comprehensive Income [Abstract]
Equity	12.   Equity   Preferred Stock   In March 2011, the remaining 8,221 shares of our Series A Preferred Stock were converted into 493,260 shares of common stock by the holder pursuant to the conversion terms of the Series A Preferred Stock. As of June 30, 2011, there are no shares of preferred stock issued and outstanding.   Share Repurchases   In February 2011, our Board of Directors authorized the repurchase of up to 20.0 million shares of our common stock. We expect to use this repurchase program principally to offset common stock issued under our share-based compensation plans. This repurchase program does not have an expiration date. Under this authorization, we repurchased approximately 2.2 million and 5.0 million shares of our common stock at a cost of  $191.3 million and  $386.6 million, respectively, during the three and six months ended June 30, 2011.   For the three and six months ended June 30, 2010, we repurchased approximately 20.8 million and 31.3 million shares at a cost of approximately  $1.0 billion and  $1.6 billion, respectively, under our 2010 and 2009 stock repurchase authorizations. We retired all of these shares as they were acquired. In connection with this retirement, in accordance with our policy, we recorded a reduction in additional paid-in-capital by the same amount.

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Comprehensive Income	6 Months Ended
Jun. 30, 2011
Equity and Comprehensive Income [Abstract]
Comprehensive Income	13.   Comprehensive Income   The following tables reflect the activity in comprehensive income included within equity attributable to the shareholders of Biogen Idec, equity attributable to noncontrolling interests, and total equity:                                                         For the Three Months     For the Three Months       Ended June 30, 2011     Ended June 30, 2010       Biogen Idec           Total     Biogen Idec           Total       Shareholders’     Noncontrolling     Shareholders’     Shareholders’     Noncontrolling     Shareholders’   (In millions)   Equity     Interests     Equity     Equity     Interests     Equity     Comprehensive income:                                                 Net income    $ 288.0      $ 16.0      $ 304.0      $ 293.4      $ 1.2      $ 294.6   Unrealized gains (losses) on securities available for sale, net of tax of  $0.6 and  $3.8     1.1       —       1.1       (6.5 )     —       (6.5 ) Unrealized gains (losses) on foreign currency forward contracts, net of tax of  $0.8 and  $3.6     6.3       —       6.3       26.5       —       26.5   Unrealized gains (losses) on pension benefit obligation, net of tax of  $0 and  $0     —       —       —       (0.2 )     —       (0.2 ) Currency translation adjustment     19.3       2.9       22.2       (71.8 )     (3.4 )     (75.2 )                                                   Comprehensive income (loss)    $ 314.7      $ 18.9      $ 333.6      $ 241.4      $ (2.2 )    $ 239.2                                                                                                             For the Six Months     For the Six Months       Ended June 30, 2011     Ended June 30, 2010       Biogen Idec           Total     Biogen Idec           Total       Shareholders’     Noncontrolling     Shareholders’     Shareholders’     Noncontrolling     Shareholders’   (In millions)   Equity     Interests     Equity     Equity     Interests     Equity     Comprehensive income:                                                 Net income    $ 582.4      $ 30.4      $ 612.8      $ 510.9      $ 3.8      $ 514.7   Unrealized gains (losses) on securities available for sale, net of tax of  $6.3 and  $5.5     (10.7 )     —       (10.7 )     (9.4 )     —       (9.4 ) Unrealized gains (losses) on foreign currency forward contracts, net of tax of  $1.2 and  $6.5     (11.1 )     —       (11.1 )     54.3       —       54.3   Unrealized gains (losses) on pension benefit obligation, net of tax of  $0 and  $0     —       —       —       (0.3 )     —       (0.3 ) Currency translation adjustment     69.1       5.7       74.8       (124.2 )     (6.0 )     (130.2 )                                                   Comprehensive income (loss)    $ 629.7      $ 36.1      $ 665.8      $ 431.3      $ (2.2 )    $ 429.1                                                         The following table reconciles equity attributable to noncontrolling interests:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Noncontrolling interests, beginning of period    $ 70.1      $ 41.2      $ 52.9      $ 40.4   Net income attributable to noncontrolling interests     16.0       1.2       30.4       3.8   Translation adjustments     2.9       (3.4 )     5.7       (6.0 ) Distributions to noncontrolling interests     (9.9 )     —       (9.9 )     —   Capital contributions from noncontrolling interests     —       1.4       —       2.2                                   Noncontrolling interests, end of period    $ 79.1      $ 40.4      $ 79.1      $ 40.4                                     Total distributions to us from our joint ventures were negligible for the three and six months ended June 30, 2011 and 2010.

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Earnings per Share	6 Months Ended
Jun. 30, 2011
Earnings per Share [Abstract]
Earnings per Share	14.   Earnings per Share   Basic and diluted earnings per share are calculated as follows:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Numerator:                                 Net income attributable to Biogen Idec Inc    $ 288.0      $ 293.4      $ 582.4      $ 510.9   Adjustment for net income allocable to preferred stock     —       (0.6 )     (0.5 )     (0.9 )                                 Net income used in calculating basic and diluted earnings per share    $ 288.0      $ 292.8      $ 581.9      $ 510.0                                   Denominator:                                 Weighted average number of common shares outstanding     242.4       259.9       241.9       265.0   Effect of dilutive securities:                                 Stock options and employee stock purchase plan     1.0       0.8       1.3       0.9   Time-vested restricted stock units     1.4       1.0       1.5       1.4   Market stock units     0.2       —       0.2       —   Performance-vested restricted stock units settled in shares     —       —       —       —                                   Dilutive potential common shares     2.6       1.8       3.0       2.3                                   Shares used in calculating diluted earnings per share     245.0       261.7       244.9       267.3                                       The following amounts were not included in the calculation of net income per diluted share because their effects were anti-dilutive:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Numerator:                                 Net income allocable to preferred stock    $ —      $ 0.6      $ 0.5      $ 0.9                                     Denominator:                                 Stock options     —       5.3       —       5.1   Time-vested restricted stock units     —       1.4       —       1.0   Market stock units     —       —       —       —   Performance-vested restricted stock units settled in shares     —       —       —       —   Convertible preferred stock     —       0.5       0.2       0.5                                     Total     —       7.2       0.2       6.6

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Share-based Payments	6 Months Ended
Jun. 30, 2011
Share-based Payments [Abstract]
Share-based Payments	15.   Share-based Payments   The following table summarizes our equity grants to employees, officers and directors under our current stock plans:                         For the Six Months     Ended June 30,     2011   2010   Stock options     —       124,000   Market stock units(a)     373,000       334,000   Cash settled performance shares(b)     483,000       373,000   Time-vested restricted stock units(c)     1,303,000       1,700,000   Performance-vested restricted stock units(d)     1,000       4,000       (a) Market stock units (MSUs) granted for the six months ended June 30, 2010, represents the target number of shares eligible to be earned at the time of grant.   MSUs granted for the six months ended June 30, 2011, includes approximately 18,000 additional MSUs issued in 2011 based upon the attainment of performance criteria set for 2010 in relation to shares granted in 2010. The remainder of the MSUs granted in 2011 represent the target number of shares eligible to be earned at the time of grant. These grants were made in conjunction with the hiring of employees and our annual awards made in February.   (b) Cash settled performance shares (CSPSs) granted for the six months ended June 30, 2010, represents the target number of shares eligible to be earned at the time of grant.   CSPSs granted for the six months ended June 30, 2011, includes approximately 95,000 additional CSPSs issued in 2011 based upon the attainment of performance criteria set for 2010 in relation to shares granted in 2010. The remainder of the CSPSs granted in 2011 represent the target number of shares eligible to be earned at the time of grant. These grants were made in conjunction with the hiring of employees and our annual awards made in February.   (c) Time-vested restricted stock units (RSUs) granted for the six months ended June 30, 2011, includes approximately 1.2 million RSUs granted in connection with our annual awards made in February 2011, and 135,000 RSUs granted in conjunction with new hires and grants made to our Board of Directors.   (d) Performance-vested restricted stock units (PVRSUs) granted for the six months ended June 30, 2010, represents the target number of shares eligible to be earned at the time of grant; approximately 1,000 additional PVRSUs were issued in 2011 based upon the attainment of performance criteria set for 2010 in relation to shares granted in 2010.     In addition, for the six months ended June 30, 2011, approximately 316,000 shares were issued under the ESPP compared to approximately 335,000 shares issued in the prior year comparative period.   The following table summarizes share-based compensation expense included within our condensed consolidated statements of income:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Research and development    $ 13.9      $ 15.4      $ 32.2      $ 32.1   Selling, general and administrative     22.2       34.0       42.8       70.2   Restructuring charges     —       —       (0.6 )     —                                     Subtotal     36.1       49.4       74.4       102.3   Capitalized share-based compensation costs     (1.0 )     (0.7 )     (2.0 )     (1.6 )                                   Share-based compensation expense included in total cost and expenses     35.1       48.7       72.4       100.7   Income tax effect     (10.9 )     (15.7 )     (23.0 )     (32.4 )                                   Share-based compensation expense included in net income attributable to Biogen Idec Inc    $ 24.2      $ 33.0      $ 49.4      $ 68.3                                       The following table summarizes share-based compensation expense associated with each of our share-based compensation programs:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Stock options    $ 1.6      $ 9.3      $ 2.7      $ 20.1   Market stock units     4.3       1.9       7.7       5.5   Time-vested restricted stock units     19.1       32.3       46.2       65.8   Performance-vested restricted stock units settled in shares     0.3       1.3       0.7       3.7   Cash settled performance shares     10.7       4.2       15.5       5.2   Employee stock purchase plan     0.1       0.4       1.6       2.0                                     Subtotal     36.1       49.4       74.4       102.3   Capitalized share-based compensation costs     (1.0 )     (0.7 )     (2.0 )     (1.6 )                                   Share-based compensation expense included in total cost and expenses    $ 35.1      $ 48.7      $ 72.4      $ 100.7

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Income Taxes	6 Months Ended
Jun. 30, 2011
Income Taxes [Abstract]
Income Taxes	16.   Income Taxes   For the three and six months ended June 30, 2011, our effective tax rate was 23.8% and 25.7%, respectively, compared to 25.8% and 25.7%, respectively, in the prior year comparative periods.   The decrease in our tax rate for the three and six months ended June 30, 2011, compared to the same periods in 2010, was primarily due to an increase in research and development expenditures eligible for the orphan drug credit and a lower effective state tax rate resulting from a change in state law, offset by a higher percentage of our 2011 profits being earned in higher tax rate jurisdictions, principally the U.S. In addition, our effective tax rate was favorably impacted by the settlement of an outstanding IRS audit matter in the first quarter of 2011.   Reconciliation between the U.S. federal statutory tax rate and our effective tax rate is summarized as follows:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,       2011     2010     2011     2010     Statutory rate     35.0 %     35.0 %     35.0 %     35.0 % State taxes     0.4       1.9       1.4       1.9   Taxes on foreign earnings     (7.3 )     (10.3 )     (6.3 )     (10.1 ) Credits and net operating loss utilization     (4.1 )     (1.6 )     (3.1 )     (1.6 ) Purchased intangible assets     1.4       1.5       1.4       1.5   IPR&D     —       0.8       —       0.8   Permanent items     (1.1 )     (1.7 )     (1.2 )     (1.7 ) Other     (0.5 )     0.2       (1.5 )     (0.1 )                                   Effective tax rate     23.8 %     25.8 %     25.7 %     25.7 %                                     Accounting for Uncertainty in Income Taxes   We and our subsidiaries are routinely examined by various taxing authorities. We file income tax returns in the U.S. federal jurisdiction, and various states and foreign jurisdictions. With few exceptions, we are no longer subject to U.S. federal tax examination for years before 2007 or state, local, or non-U.S. income tax examinations by tax authorities for years before 2001. During the second quarter of 2011, we adjusted our unrecognized tax benefits to reflect new information arising during our ongoing audit examinations.   Contingencies   In 2006, the Massachusetts Department of Revenue (DOR) issued a Notice of Assessment against Biogen Idec MA Inc. (BIMA), one of our wholly-owned subsidiaries, for  $38.9 million of corporate excise tax for 2002, which includes associated interest and penalties. The assessment asserts that the portion of sales attributable to Massachusetts (sales factor), the computation of BIMA’s research and development credits and certain deductions claimed by BIMA were not appropriate, resulting in unpaid taxes for 2002. We filed an abatement application with the DOR seeking abatement for 2001, 2002 and 2003. Our abatement application was denied and on July 25, 2007 we filed a petition with the Massachusetts Appellate Tax Board (the Massachusetts ATB) seeking, among other items, abatements of corporate excise tax for 2001, 2002 and 2003 and adjustments in certain credits and credit carryforwards for 2001, 2002 and 2003. Issues before the Massachusetts ATB include the computation of BIMA’s sales factor for 2001, 2002 and 2003, computation of BIMA’s research credits for those same years, and the availability of deductions for certain expenses and partnership flow-through items. The hearing on our petition has been stayed by the Massachusetts ATB to allow the parties to discuss a negotiated resolution of all disputes as to 2001, 2002 and 2003. The Massachusetts ATB has ordered a status conference for September 6, 2011, at which the parties will report on the status of the settlement discussions. If a negotiated resolution is concluded, we do not expect it to have a significant impact on our financial position or results of operations. We have and will continue to evaluate the facts, circumstances and information available in accordance with our financial reporting policies to reflect management’s best estimate of the outcome. Based upon our most recent estimates, we currently expect to settle this matter in exchange for a payment of  $7.0 million in taxes, plus interest, and expect to reach an agreement on the tax credits carried forward into 2004.   On June 8, 2010, we received Notices of Assessment from the DOR against BIMA for  $103.5 million of corporate excise tax, including associated interest and penalties, related to our 2004, 2005 and 2006 tax filings. We believe the asserted basis for these assessments is consistent with that for 2002. Assessments related to periods under dispute, including associated interest and penalties, total  $142.4 million. We filed an abatement application with the DOR seeking abatement for 2004, 2005 and 2006. Our abatement application was denied and we filed a petition appealing the denial with the ATB on February 3, 2011. For all periods under dispute, we believe that positions taken in our tax filings are valid and believe that we have meritorious defenses in these disputes. We are contesting these matters vigorously.   Our tax filings for 2007 and 2008 have not yet been audited by the DOR but have been prepared in a manner consistent with prior filings which may result in an assessment for those years. Due to tax law changes effective January 1, 2009, the computation and deductions at issue in previous tax filings have not been part of our tax filings in Massachusetts starting in 2009.   We believe that these assessments do not impact the level of liabilities for income tax contingencies. However, there is a possibility that we may not prevail in defending all of our assertions with the DOR. If these matters are resolved unfavorably in the future, the resolution could have a material adverse impact on the effective tax rate and our results of operations.

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Other Consolidated Financial Statement Detail	6 Months Ended
Jun. 30, 2011
Business and Collaborations [Abstract]
Other Consolidated Financial Statement Detail	17.   Other Consolidated Financial Statement Detail   Other Income (Expense), Net   Components of other income (expense), net, are summarized as follows:                                         For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Interest income    $ 4.3      $ 6.7      $ 8.0      $ 15.6   Interest expense     (8.4 )     (9.0 )     (17.6 )     (17.3 ) Impairments of investments     (5.5 )     (1.2 )     (6.8 )     (17.0 ) Foreign exchange gains (losses), net     (0.6 )     (0.7 )     (1.0 )     0.3   Gain (loss) on sales of investments, net     0.2       6.3       15.5       11.3   Other, net     (1.7 )     (1.1 )     0.1       (0.3 )                                   Total other income (expense), net    $ (11.7 )    $ 1.0      $ (1.8 )    $ (7.4 )                                       Other Current Assets   Other current assets consist of the following:                         As of     As of       June 30,     December 31,   (In millions)   2011     2010     Deferred tax assets    $ 66.9      $ 112.2   Prepaid taxes     33.1       31.4   Receivable from collaborations     8.8       7.3   Interest receivable     6.1       4.9   Other prepaid expenses     60.1       47.9   Other     26.8       12.1                     Total other current assets    $ 201.8      $ 215.8                       Accrued Expenses and Other   Accrued expenses and other consists of the following:                         As of     As of       June 30,     December 31,   (In millions)   2011     2010     Employee compensation and benefits    $ 134.3      $ 159.7   Revenue-related rebates     112.4       105.3   Restructuring charges     8.9       66.4   Royalties and licensing fees     40.0       45.1   Deferred revenue     59.1       41.3   Collaboration expenses     57.1       31.6   Clinical development expenses     33.7       24.4   Interest payable     21.6       21.6   Construction in progress acrual     13.3       16.4   Current portion of contingent consideration     4.9       11.9   Derivative liability     24.9       12.2   Other     131.1       130.0                     Total accrued expenses and other    $ 641.3      $ 665.9                       For a discussion of restructuring charges accrued as of June 30, 2011 and December 31, 2010, please read Note 3, Restructuring to these condensed consolidated financial statements.

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Investments in Variable Interest Entities	6 Months Ended
Jun. 30, 2011
Investments in Variable Interest Entities [Abstract]
Investments in Variable Interest Entities	18.   Investments in Variable Interest Entities   Consolidated Variable Interest Entities   Our condensed consolidated financial statements include the financial results of variable interest entities in which we are the primary beneficiary.   Investments in Joint Ventures   We consolidate 100% of the operations of Biogen Dompé SRL and Biogen Dompé Switzerland GmbH, our respective sales affiliates in Italy and Switzerland, as we retain the contractual power to direct the activities of these entities which most significantly and directly impact their economic performance. The activity of each of these joint ventures is significant to our overall operations. The assets of these joint ventures were restricted, from the standpoint of Biogen Idec, in that they are not available for our general business use outside the context of each joint venture. The holders of the liabilities of each joint venture, including the credit line from Dompé described in our 2010 Form 10-K, had no recourse to Biogen Idec. Other than the line of credit from us and Dompé Farmaceutici SpA to Biogen-Dompé SRL, we have provided no financing to these joint ventures. In addition, Biogen-Dompé SRL has an operating lease for office space as well as a contract for the provision of administrative services with Dompé Farmaceutici SpA.   The following table summarizes total joint venture assets and liabilities:                         As of   As of     June 30,   December 31, (In millions)   2011   2010   Assets    $ 193.6      $ 159.2   Liabilities    $ 74.6      $ 63.3     The joint ventures’ most significant assets were accounts receivable from the ordinary course of business. As of June 30, 2011, accounts receivable held by our joint ventures totaled  $134.2 million, of which  $127.8 million were related to Biogen Dompé SRL, compared to  $124.2 million as of December 31, 2010, of which  $118.0 million were related to Biogen Dompé SRL. For additional information related to our accounts receivable balances in Italy, please read Note 5, Accounts Receivable to these condensed consolidated financial statements.   Knopp   In August 2010, we entered into a license agreement with Knopp Neurosciences, Inc. (Knopp), a subsidiary of Knopp Holdings, LLC, for the development, manufacture and commercialization of dexpramipexole, an orally administered small molecule in clinical development for the treatment of amyotrophic lateral sclerosis (ALS). We are responsible for all development activities and, if successful, we will also be responsible for the manufacture and global commercialization of dexpramipexole. Under the terms of the license agreement we made a  $26.4 million upfront payment and agreed to pay Knopp up to an additional  $265.0 million in development and sales-based milestone payments, as well as royalties on future commercial sales. In addition, we also purchased 30.0% of the Class B common shares of Knopp for  $60.0 million.   Due to the terms of the license agreement and our investment in Knopp, we determined that we are the primary beneficiary of Knopp as we have the power to direct the activities that most significantly impact Knopp’s economic performance. As such, we consolidate the results of Knopp. Although we have assumed responsibility for the development of dexpramipexole, we may also be required to reimburse certain Knopp expenses directly attributable to the license agreement. Any additional amounts incurred by Knopp that we reimburse will be reflected within total costs and expenses in our consolidated statement of income. Future development and sales-based milestone payments will be reflected within our consolidated statement of income as charges to noncontrolling interests when such milestones are achieved.   In March 2011, we dosed the first patient in a registrational study for dexpramipexole. The achievement of this milestone resulted in a  $10.0 million payment due to Knopp. As we consolidate Knopp, we recognized this payment as a charge to noncontrolling interests in the first quarter of 2011.   For the three and six months ended June 30, 2011, the collaboration incurred  $9.0 million and  $14.7 million, respectively, of expense related to the development of dexpramipexole, which is reflected as research and development expense within our condensed consolidated statements of income. The assets and liabilities of Knopp are not significant to our financial position or results of operations. We have provided no financing to Knopp other than previously contractually required amounts disclosed above.   Neurimmune SubOne AG   In 2007, we entered into a collaboration agreement with Neurimmune SubOne AG (Neurimmune), a subsidiary of Neurimmune AG, for the development and commercialization of antibodies for the treatment of Alzheimer’s disease. Neurimmune conducts research to identify potential therapeutic antibodies and we are responsible for the development, manufacturing and commercialization of all products. Based upon our current development plans, we may pay Neurimmune up to  $345.0 million in remaining milestone payments, as well as royalties on sales of any resulting commercial products.   We determined that we are the primary beneficiary of Neurimmune because we have the power through the collaboration to direct the activities that most significantly impact SubOne’s economic performance and are required to fund 100% of the research and development costs incurred in support of the collaboration agreement. Amounts that are incurred by Neurimmune for research and development expenses in support of the collaboration that we reimburse are reflected in research and development expense in our consolidated statements of income. Future milestone payments will be reflected within our consolidated statements of income as a charge to the noncontrolling interest when such milestones are achieved.   For the three and six months ended June 30, 2011, the collaboration incurred development expense totaling  $3.0 million and  $4.8 million, respectively, which is reflected as research and development expense within our condensed consolidated statements of income, compared to  $5.3 million and  $10.4 million, respectively, in the prior year comparative periods.   In April 2011, we submitted an Investigational New Drug (IND) application for BIIB037 (human anti-Amyloid β mAb), a beta-amyloid removal therapy. BIIB037 is being developed for the treatment of Alzheimer’s disease. The achievement of this milestone resulted in a  $15.0 milestone payment made to Neurimmune. As we consolidate Neurimmune, we have recognized this payment as a charge to noncontrolling interests in the second quarter of 2011.   The assets and liabilities of Neurimmune are not significant as it is a research and development organization. We have provided no financing to Neurimmune other than previously contractually required amounts disclosed above.   Unconsolidated Variable Interest Entities   We have relationships with other variable interest entities which we do not consolidate as we lack the power to direct the activities that significantly impact the economic success of these entities. These relationships include investments in certain biotechnology companies and research collaboration agreements. For additional information related to our significant collaboration arrangements, please read Note 19, Collaborations to our consolidated financial statements included within our 2010 Form 10-K.   As of June 30, 2011 and December 31, 2010, the total carrying value of our investments in biotechnology companies that we determined to be variable interest entities and which are not consolidated were  $17.7 million and  $22.9 million, respectively. Our maximum exposure to loss related to these variable interest entities is limited to the carrying value of our investments.   We have entered into research collaborations with certain variable interest entities where we are required to fund certain development activities. These development activities are included in research and development expense within our consolidated statements of income as they are incurred. Depending on the collaborative arrangement, we may record funding receivables or payable balances with our partners, based on the nature of the cost-sharing mechanism and activity within the collaboration. As of June 30, 2011 and December 31, 2010, we had no significant receivables or payables related to cost sharing arrangements with unconsolidated variable interest entities.   We have provided no financing to these variable interest entities other than previously contractually required amounts.

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Collaborations	6 Months Ended
Jun. 30, 2011
Business and Collaborations [Abstract]
Collaborations	19.   Collaborations   In April 2011, we agreed to terminate our collaboration with Vernalis plc. (Vernalis) for the development and commercialization of an adenosine A2a receptor antagonist for treatment of Parkinson’s disease effective April 11, 2011. Under the terms of the agreement, we will return the program to Vernalis and have no further license to, or continuing involvement in the development of, this compound and its related intellectual property. In exchange, we will receive a royalty on future net sales if this compound is ultimately commercialized. We funded development costs through the termination date and have no other remaining development obligations after that date. Development expense incurred by this collaboration in 2011 was insignificant.

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Litigation	6 Months Ended
Jun. 30, 2011
Litigation [Abstract]
Litigation	20.   Litigation   Massachusetts Department of Revenue   In 2006, the Massachusetts Department of Revenue (DOR) issued a Notice of Assessment against Biogen Idec MA, Inc. (BIMA) for  $38.9 million of corporate excise tax for 2002, which includes associated interest and penalties. The assessment asserts that the portion of sales attributable to Massachusetts (sales factor), the computation of BIMA’s research and development credits and certain deductions claimed by BIMA were not appropriate, resulting in unpaid taxes for 2002. We filed an abatement application with the DOR seeking abatements for 2001, 2002 and 2003. Our abatement application was denied and on July 25, 2007, we filed a petition with the Massachusetts Appellate Tax Board (the Massachusetts ATB) seeking, among other items, abatements of corporate excise tax for 2001, 2002 and 2003 and adjustments in certain credits and credit carry forwards for 2001, 2002 and 2003. Issues before the Massachusetts ATB include the computation of BIMA’s sales factor for 2001, 2002 and 2003, computation of BIMA’s research credits for those same years, and the availability of deductions for certain expenses and partnership flow-through items. The hearing on our petition has been stayed by the Massachusetts ATB to allow the parties to discuss a negotiated resolution of all disputes as to 2001, 2002 and 2003. The Massachusetts ATB has ordered a status conference for September 6, 2011, at which the parties will report on the status of the settlement discussions.   On June 8, 2010, we received Notices of Assessment from the DOR against BIMA for  $103.5 million of corporate excise tax, including associated interest and penalties, related to our 2004, 2005 and 2006 tax filings. We believe the asserted basis for these assessments is consistent with that for 2002. We filed an abatement application with the DOR seeking abatements for 2004, 2005, and 2006. Our abatement application was denied on December 15, 2010 and we filed a petition appealing the denial with the Massachusetts ATB on February 3, 2011. For all periods under dispute, we believe that positions taken in our tax filings are valid and believe that we have meritorious defenses in these disputes. We are contesting these matters vigorously.   Hoechst — Genentech Arbitration   On October 24, 2008, Hoechst GmbH (“Hoechst”), predecessor to Sanofi-Aventis Deutschland GmbH (“Sanofi”), filed with the ICC International Court of Arbitration (Paris) a request for arbitration against Genentech, relating to a license agreement (the “Hoechst License”) between Hoechst’s predecessor and Genentech that was entered as of January 1, 1991 and terminated by Genentech on October 27, 2008. The Hoechst License granted Genentech certain rights with respect to U.S. Patents 5,849,522 (’522 patent) and 6,218,140 (’140 patent) and related patents outside the U.S. The license agreement provided for royalty payments of 0.5% on net sales of certain products defined by the agreement. In June 2011, the arbitrator issued an intermediate decision indicating that RITUXAN is such a product and ordering Genentech to provide certain RITUXAN sales information for the period from December 15, 1998 to October 27, 2008. The arbitrator will use this information to ascertain the amount of damages to be awarded to Hoechst. Genentech has filed a Declaration of Appeal from the intermediate decision in the Court of Appeal in Paris, which is pending. Although we are not a party to the arbitration, we expect that certain damages that may be awarded to Hoechst will be a cost charged to our collaboration with Genentech. Accordingly, we have reduced our share of RITUXAN revenues from unconsolidated joint business by approximately  $50.0 million in the second quarter of 2011, as a result of an accrual for estimated compensatory damages (including interest) relating to the arbitrator’s intermediate decision. We expect the impact in subsequent quarters will be limited to adjustments necessary to reflect the difference between our estimate and the damages attributable to our collaboration or a successful challenge by Genentech of the arbitrator’s decision.   Sanofi ’522 and ’140 Patent Litigation   On October 27, 2008, Sanofi, successor to Hoechst, filed suit against Genentech and Biogen Idec in federal court in Texas (E.D. Tex.) (Texas Action) claiming that RITUXAN and certain other Genentech products infringe the ’522 patent and the ’140 patent. The patents are due to expire in December 2015. Sanofi seeks preliminary and permanent injunctions, compensatory and exemplary damages, and other relief. The same day Genentech and Biogen Idec filed a complaint against Sanofi in federal court in California (N.D. Cal.) (California Action) seeking a declaratory judgment that RITUXAN and other Genentech products do not infringe the ’522 patent or the ’140 patent and a declaratory judgment that those patents are invalid. The Texas Action was ordered transferred to the federal court in the Northern District of California and consolidated with the California Action and we refer to the two actions together as the Consolidated Sanofi Patent Actions. On April 21, 2011, the court entered a separate and final judgment that the manufacture and sale of RITUXAN do not infringe the ’522 patent or the ’140 patent and stayed the trial of the remaining claims, including Biogen Idec’s and Genentech’s invalidity claims. Sanofi has filed a notice of appeal from the court’s non-infringement ruling to the U.S. Court of Appeals for the Federal Circuit. We have not formed an opinion that a decision in favor of Sanofi in its appeal of the non-infringement ruling, or an unfavorable outcome on the now stayed invalidity claims in the Consolidated Sanofi Patent Actions, is either “probable” or “remote.” We believe that we have good and valid defenses and are vigorously defending against Sanofi’s allegations. In the event that we and Genentech are found liable we estimate that the range of any potential loss could extend to a royalty of up to 0.5% of net sales of RITUXAN, based on, among other things, the royalty rate set forth in the terminated Hoechst License and an analysis of royalty rates charged for comparable technologies. We believe that Sanofi would seek a substantially higher royalty rate, and we will continue to vigorously oppose its claims and position. One of the issues to be resolved in the Consolidated Sanofi Patent Actions is whether any award of reasonable royalty damages would begin running from October 27, 2008, when Genentech terminated the Hoechst License, or from October 27, 2002, six years before Sanofi filed the Texas Action, the statutory limitations period for damages in patent cases. In the event that Genentech is ordered in the arbitration described above to pay royalties on RITUXAN sales under the Hoechst License up to the date of the termination of the Hoechst License (October 27, 2008), we do not anticipate that either we or Genentech would be subject to any damages award in the Consolidated Sanofi Patent Actions for any period before October 27, 2008. Certain damages that may be awarded to Sanofi may be a cost charged to our collaboration with Genentech.   ’755 Patent Litigation   On September 15, 2009, we were issued U.S. Patent No. 7,588,755 (’755 Patent), which claims the use of interferon beta for immunomodulation or treating a viral condition, viral disease, cancers or tumors. This patent, which expires in September 2026, covers, among other things, the treatment of MS with our product AVONEX. On May 27, 2010, Bayer Healthcare Pharmaceuticals Inc. (Bayer) filed a lawsuit against us in the U.S. District Court for the District of New Jersey seeking a declaratory judgment of patent invalidity and noninfringement and seeking monetary relief in the form of attorneys’ fees, costs and expenses. On May 28, 2010, BIMA filed a lawsuit in the U.S. District Court for the District of New Jersey alleging infringement of the ’755 Patent by EMD Serono, Inc. (manufacturer, marketer and seller of REBIF), Pfizer, Inc. (co-marketer of REBIF), Bayer (manufacturer, marketer and seller of BETASERON and manufacturer of EXTAVIA), and Novartis Pharmaceuticals Corp. (marketer and seller of EXTAVIA) and seeking monetary damages, including lost profits and royalties. The court has consolidated the two lawsuits, and we refer to the two actions as the Consolidated ‘755 Patent Actions. On August 16, 2010, BIMA amended its complaint to add Ares Trading S.A. (Ares), an affiliate of EMD Serono, as a defendant, and to seek a declaratory judgment that a purported “nonsuit and option agreement” between Ares and BIMA dated October 12, 2000, that purports to provide that Ares will have an option to obtain a license to the ’755 Patent, is not a valid and enforceable agreement or, alternatively, has been revoked and/or terminated by the actions of Ares or its affiliates. Ares moved to compel arbitration of the claims against it, and on June 7, 2011, a United States Magistrate Judge recommended allowance of Ares’ motion. On June 21, 2011, we filed objections to the recommendation. Pending a decision on our objections by the U.S. District Court Judge, an arbitration tribunal has convened and has scheduled a hearing for October 19-21, 2011. Bayer, Pfizer, Novartis and EMD Serono have all filed counterclaims in the Consolidated ‘755 Patent Actions seeking declaratory judgments of patent invalidity and noninfringement, and seeking monetary relief in the form of costs and attorneys’ fees, and EMD Serono and Bayer have each filed a counterclaim seeking a declaratory judgment that the ’755 Patent is unenforceable based on alleged inequitable conduct. Bayer has also amended its complaint to seek such a declaration. No trial date has yet been ordered, but we expect that the trial of the Consolidated ‘755 Patent Actions will take place in 2013.   GSK ’612 Patent Litigation   On March 23, 2010, we and Genentech were issued U.S. Patent No. 7,682,612 (‘612 Patent) relating to a method of treating CLL using an anti-CD20 antibody. The patent which expires in November 2019 covers, among other things, the treatment of CLL with RITUXAN. On March 23, 2010, we filed a lawsuit in federal court in the Southern District of California against Glaxo Group Limited and GlaxoSmithKline LLC (collectively, GSK) alleging infringement of that patent based upon GSK’s manufacture, marketing and sale, offer to sell, and importation of ARZERRA. We seek damages, including a royalty and lost profits, and injunctive relief. GSK has filed a counterclaim seeking a declaratory judgment of patent invalidity, noninfringement, unenforceability, and inequitable conduct, and seeking monetary relief in the form of costs and attorneys’ fees.   Novartis V&D ’688 Patent Litigation   On January 26, 2011, Novartis Vaccines and Diagnostics, Inc. (Novartis V&D) filed suit against us in federal district court in Delaware, alleging that TYSABRI infringes U.S. Patent No. 5,688,688 “Vector for Expression of a Polypeptide in a Mammalian Cell” (‘688 Patent), which was granted in November 1997 and expires in November 2014. Novartis V&D seeks a declaration of infringement, a finding of willful infringement, compensatory damages, treble damages, interest, costs and attorneys’ fees. We have not formed an opinion that an unfavorable outcome is either “probable” or “remote”, and are unable to estimate the magnitude or range of any potential loss. We believe that we have good and valid defenses to the complaint and will vigorously defend against it.   Product Liability and Other Legal Proceedings   We are also involved in product liability claims and other legal proceedings generally incidental to our normal business activities. While the outcome of any of these proceedings cannot be accurately predicted, we do not believe the ultimate resolution of any of these existing matters would have a material adverse effect on our business or financial conditions.

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Segment Information	6 Months Ended
Jun. 30, 2011
Segment Information [Abstract]
Segment Information	21.   Segment Information   We operate as one business segment, which is the business of discovering, developing, manufacturing and marketing products for the treatment of serious diseases with a focus on neurological disorders and therefore, our chief operating decision-maker manages the operations of our Company as a single operating segment.

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New Accounting Pronouncements	6 Months Ended
Jun. 30, 2011
New Accounting Pronouncements [Abstract]
New Accounting Pronouncements	22.   New Accounting Pronouncements   From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other standard setting bodies that are adopted by the Company as of the specified effective date. Unless otherwise discussed, we believe that the impact of recently issued standards that are not yet effective will not have a material impact on our financial position or results of operations upon adoption.   In June 2011, the FASB issued Accounting Standards Update (ASU) No. 2011-05, “Comprehensive Income (Topic 220)” (ASU 2011-05). This newly issued accounting standard (1) eliminates the option to present the components of other comprehensive income as part of the statement of changes in stockholders’ equity; (2) requires the consecutive presentation of the statement of net income and other comprehensive income; and (3) requires an entity to present reclassification adjustments on the face of the financial statements from other comprehensive income to net income. The amendments in this ASU do not change the items that must be reported in other comprehensive income or when an item of other comprehensive income must be reclassified to net income nor do the amendments affect how earnings per share is calculated or presented. This ASU is required to be applied retrospectively and is effective for fiscal years and interim periods within those years beginning after December 15, 2011, which for Biogen Idec means January 1, 2012. As this accounting standard only requires enhanced disclosure, the adoption of this standard will not impact our financial position or results of operations.   In May 2011, the FASB issued ASU No. 2011-04, “Fair Value Measurement (Topic 820): Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in U.S. GAAP and IFRSs” (ASU 2011-04). This newly issued accounting standard clarifies the application of certain existing fair value measurement guidance and expands the disclosures for fair value measurements that are estimated using significant unobservable (Level 3) inputs. This ASU is effective on a prospective basis for annual and interim reporting periods beginning on or after December 15, 2011, which for Biogen Idec means January 1, 2012. We do not expect that adoption of this standard will have a material impact on our financial position or results of operations.   In January 2010, we adopted a newly issued accounting standard which requires additional disclosure about the amounts of and reasons for significant transfers in and out of Level 1 and Level 2 fair value measurements. This standard also clarified existing disclosure requirements related to the level of disaggregation of fair value measurements for each class of assets and liabilities and requires disclosures about inputs and valuation techniques used to measure fair value for both recurring and nonrecurring Level 2 and Level 3 measurements. In addition, effective for interim and annual periods beginning after December 15, 2010, which for Biogen Idec is January 1, 2011, this standard further requires an entity to present disaggregated information about activity in Level 3 fair value measurements on a gross basis, rather than as one net amount. As this newly issued accounting standard only requires enhanced disclosure, the adoption of this standard did not impact our financial position or results of operations.

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Business (Policies)	6 Months Ended
Jun. 30, 2011
Business and Collaborations [Abstract]
Overview	Biogen Idec is a global biotechnology company focused on discovering, developing, manufacturing and marketing products for the treatment of serious diseases with a focus on neurological disorders. We currently have four marketed products: AVONEX, RITUXAN, TYSABRI, and FUMADERM. Our marketed products are used for the treatment of multiple sclerosis (MS), non-Hodgkin’s lymphoma (NHL), rheumatoid arthritis (RA), Crohn’s disease, chronic lymphocytic leukemia (CLL), and psoriasis.
Basis of Presentation	In the opinion of management, the accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair presentation of our financial statements for interim periods in accordance with accounting principles generally accepted in the United States (U.S. GAAP). The information included in this quarterly report on Form 10-Q should be read in conjunction with our consolidated financial statements and the accompanying notes included in our Annual Report on Form 10-K for the year ended December 31, 2010 (2010 Form 10-K). Our accounting policies are described in the “Notes to Consolidated Financial Statements” in our 2010 Form 10-K and updated, as necessary, in this Form 10-Q. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from audited financial statements, but does not include all disclosures required by U.S. GAAP. The results of operations for the three and six months ended June 30, 2011 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.
Consolidation	Our condensed consolidated financial statements reflect our financial statements, those of our wholly-owned subsidiaries and those of certain variable interest entities in which we are the primary beneficiary. For consolidated entities in which we own less than a 100% interest, we record net income (loss) attributable to noncontrolling interests in our consolidated statement of income equal to the percentage of the economic or ownership interest retained in such entities by the respective noncontrolling parties. All material intercompany balances and transactions have been eliminated in consolidation.   In determining whether we are the primary beneficiary of an entity, we apply a qualitative approach, that determines whether we have both (1) the power to direct the economically significant activities of the entity and (2) the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to that entity. These considerations impact the way we account for our existing collaborative and joint venture relationships and determine whether we consolidate companies or entities with which we have collaborative or other arrangements. Determination about whether an enterprise should consolidate a variable interest entity is required to be evaluated continuously as changes to existing relationships or future transactions may result in us consolidating or deconsolidating our partner(s) to collaborations and other arrangements.
Use of Estimates	The preparation of our condensed consolidated financial statements in accordance with U.S. GAAP requires management to make estimates, judgments, and assumptions that may affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosure of contingent assets and liabilities. On an on-going basis, we evaluate our estimates and judgments and methodologies, including those related to revenue recognition and related allowances, our collaborative relationships, clinical trial expenses, the consolidation of variable interest entities, the valuation of contingent consideration resulting from a business combination, the valuation of acquired intangible assets including in-process research and development, inventory, impairment and amortization of long-lived assets including intangible assets, impairments of goodwill, share-based compensation, income taxes including the valuation allowance for deferred tax assets, valuation of investments, derivatives and hedging activities, contingencies, litigation, and restructuring charges. We base our estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results may differ from these estimates under different assumptions or conditions.
Subsequent Events	We did not have any material recognizable subsequent events. However, we did have the following non-recognizable subsequent events:     •  On July 20, 2011, the European Commission (EC) granted a conditional marketing authorisation for FAMPYRA (prolonged release fampridine) in the E.U., which triggered a  $25.0 million milestone payment payable to Acorda Therapeutics, Inc. (Acorda). FAMPYRA is an oral compound indicated as a treatment to improve walking ability in people with MS.     •  On July 14, 2011, we executed leases for two office buildings to be built in Cambridge, Massachusetts. These buildings, totaling approximately 500,000 square feet, will serve as the future location of our corporate headquarters and commercial operations. The buildings will also provide additional general and administrative and research and development office space. For a more detailed description of these transactions, please read Note 11, Property, Plant and Equipment to these condensed consolidated financial statements.
Revenue recognition	We recognize revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; delivery has occurred or services have been rendered; our price to the customer is fixed or determinable; and collectibility is reasonably assured.   Product Revenues   Revenues from product sales are recognized when title and risk of loss have passed to the customer, which is typically upon delivery. However, sales of TYSABRI in the U.S. are recognized on the “sell-through” model, that is, upon shipment of the product by Elan Pharma International, Ltd. (Elan), an affiliate of Elan Corporation, plc., to its third party distributor rather than upon shipment to Elan. Product revenues are recorded net of applicable reserves for discounts and allowances.   Reserves for Discounts and Allowances   We establish reserves for trade term discounts, wholesaler incentives, Medicaid rebates, Veterans Administration (VA) and Public Health Service (PHS) discounts, managed care rebates, product returns and other governmental rebates or applicable allowances. Reserves established for these discounts and allowances are classified as reductions of accounts receivable (if the amount is payable to our direct customer) or a liability (if the amount is payable to a party other than our customer). In addition, we distribute no-charge product to qualifying patients under our patient assistance and patient replacement goods program. This program is administered through one of our distribution partners, which ships product to qualifying patients from its own inventory received from us. Gross revenue and the related reserves are not recorded on product shipped under this program and cost of sales is recorded when the product is shipped.   Revenues from Unconsolidated Joint Business   We collaborate with Genentech on the development and commercialization of RITUXAN. Revenues from unconsolidated joint business consist of (1) our share of pre-tax co-promotion profits in the U.S.; (2) reimbursement of our selling and development expense in the U.S.; and (3) revenue on sales of RITUXAN in the rest of world, which consists of our share of pretax co-promotion profits in Canada and royalty revenue on sales of RITUXAN outside the U.S. and Canada by F. Hoffmann-La Roche Ltd. (Roche) and its sublicensees. Pre-tax co-promotion profits are calculated and paid to us by Genentech in the U.S. and by Roche in Canada. Pre-tax co-promotion profits consist of U.S. and Canadian sales of RITUXAN to third-party customers net of discounts and allowances less the cost to manufacture RITUXAN, third-party royalty expenses, distribution, selling and marketing, and development expenses incurred by Genentech, Roche and us. We record our share of the pretax co-promotion profits in Canada and royalty revenues on sales of RITUXAN outside the U.S. on a cash basis. Additionally, our share of the pretax co-promotion profits in the U.S. includes estimates supplied by Genentech.   Royalty Revenues   We receive royalty revenues on sales by our licensees of other products covered under patents that we own. We do not have future performance obligations under these license arrangements. We record these revenues based on estimates of the sales that occurred during the relevant period. The relevant period estimates of sales are based on interim data provided by licensees and analysis of historical royalties that have been paid to us, adjusted for any changes in facts and circumstances, as appropriate. We maintain regular communication with our licensees in order to assess the reasonableness of our estimates. Differences between actual royalty revenues and estimated royalty revenues are adjusted in the period in which they become known, typically the following quarter. Historically, adjustments have not been material when compared to actual amounts paid by licensees. If we are ever unable to accurately estimate revenue, then we record revenues on a cash basis.
Comprehensive Income	In June 2011, the FASB issued Accounting Standards Update (ASU) No. 2011-05, “Comprehensive Income (Topic 220)” (ASU 2011-05). This newly issued accounting standard (1) eliminates the option to present the components of other comprehensive income as part of the statement of changes in stockholders’ equity; (2) requires the consecutive presentation of the statement of net income and other comprehensive income; and (3) requires an entity to present reclassification adjustments on the face of the financial statements from other comprehensive income to net income. The amendments in this ASU do not change the items that must be reported in other comprehensive income or when an item of other comprehensive income must be reclassified to net income nor do the amendments affect how earnings per share is calculated or presented. This ASU is required to be applied retrospectively and is effective for fiscal years and interim periods within those years beginning after December 15, 2011, which for Biogen Idec means January 1, 2012. As this accounting standard only requires enhanced disclosure, the adoption of this standard will not impact our financial position or results of operations.
Fair Value Measurement	In May 2011, the FASB issued ASU No. 2011-04, “Fair Value Measurement (Topic 820): Amendments to Achieve Common Fair Value Measurement and Disclosure Requirements in U.S. GAAP and IFRSs” (ASU 2011-04). This newly issued accounting standard clarifies the application of certain existing fair value measurement guidance and expands the disclosures for fair value measurements that are estimated using significant unobservable (Level 3) inputs. This ASU is effective on a prospective basis for annual and interim reporting periods beginning on or after December 15, 2011, which for Biogen Idec means January 1, 2012. We do not expect that adoption of this standard will have a material impact on our financial position or results of operations.   In January 2010, we adopted a newly issued accounting standard which requires additional disclosure about the amounts of and reasons for significant transfers in and out of Level 1 and Level 2 fair value measurements. This standard also clarified existing disclosure requirements related to the level of disaggregation of fair value measurements for each class of assets and liabilities and requires disclosures about inputs and valuation techniques used to measure fair value for both recurring and nonrecurring Level 2 and Level 3 measurements. In addition, effective for interim and annual periods beginning after December 15, 2010, which for Biogen Idec is January 1, 2011, this standard further requires an entity to present disaggregated information about activity in Level 3 fair value measurements on a gross basis, rather than as one net amount. As this newly issued accounting standard only requires enhanced disclosure, the adoption of this standard did not impact our financial position or results of operations.

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Restructuring (Tables)	6 Months Ended
Jun. 30, 2011
Restructuring [Abstract]
Restructuring liability	Workforce     Facility         (In millions)   Reduction     Consolidation     Total     Restructuring reserve as of December 31, 2010    $ 60.6      $ 5.8      $ 66.4   Expense     12.1       2.4       14.5   (Payments) receipts, net     (73.8 )     (2.0 )     (75.8 ) Adjustments to previous estimates, net     (1.6 )     —       (1.6 ) Other adjustments     8.6       (3.2 )     5.4                             Restructuring reserve as of June 30, 2011    $ 5.9      $ 3.0      $ 8.9

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Revenue Recognition (Tables)	6 Months Ended
Jun. 30, 2011
Revenue Recognition [Abstract]
Analysis of the amount of, and change in, product revenue reserves	Contractual               (In millions)   Discounts     Adjustments     Returns     Total     Balance, as of December 31, 2010    $ 13.9      $ 107.0      $ 21.1      $ 142.0   Current provisions relating to sales in current year     47.2       174.6       6.8       228.6   Adjustments relating to prior years     —       (8.4 )     —       (8.4 ) Payments/returns relating to sales in current year     (33.3 )     (101.4 )     (0.3 )     (135.0 ) Payments/returns relating to sales in prior years     (13.0 )     (58.5 )     (4.9 )     (76.4 )                                   Balance, as of June 30, 2011    $ 14.8      $ 113.3      $ 22.7      $ 150.8                                       Our product revenue reserves are based on estimates of the amounts earned or to be claimed on the related sales. These estimates take into consideration our historical experience, current contractual and statutory requirements, specific known market events and trends and forecasted customer buying patterns. Actual amounts may ultimately differ from our estimates. If actual results vary, it will result in an adjustment to these estimates, which could have an effect on earnings in the period of adjustment.   During the six months ended June 30, 2011, we reduced our reserves for contractual adjustments by  $8.4 million, which was primarily due to a revision of our previous estimates associated with the impact of healthcare reform.
Summary of total product revenue reserves included in consolidated balance sheets	As of     As of       June 30,     December 31,   (In millions)   2011     2010     Reduction of accounts receivable    $ 38.4      $ 36.7   Current liability     112.4       105.3                     Total reserves    $ 150.8      $ 142.0

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Accounts Receivable [Tables]	6 Months Ended
Jun. 30, 2011
Accounts Receivable [Abstract]
Net accounts receivable balance from different countries	As of June 30, 2011     Balance Included   Balance Included         within Accounts   within Investments     (In millions)   Receivable, Net   and Other Assets   Total   Italy    $ 116.5      $ 11.3      $ 127.8   Spain    $ 81.7      $ 36.7      $ 118.4   Portugal    $ 23.9      $ 7.4      $ 31.3   Greece    $ 10.7      $ —      $ 10.7                                   As of December 31, 2010     Balance Included   Balance Included         within Accounts   within Investments     (In millions)   Receivable, Net   and Other Assets   Total   Italy    $ 103.2      $ 14.8      $ 118.0   Spain    $ 70.8      $ 29.8      $ 100.6   Portugal    $ 17.8      $ 5.5      $ 23.3   Greece    $ 3.9      $ —      $ 3.9

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Inventory (Tables)	6 Months Ended
Jun. 30, 2011
Inventory [Abstract]
Components of inventories	As of     As of       June 30,     December 31,   (In millions)   2011     2010     Raw materials    $ 59.8      $ 59.0   Work in process     165.6       142.2   Finished goods     82.9       87.9                     Total inventory    $ 308.3      $ 289.1

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Intangible Assets and Goodwill (Tables)	6 Months Ended
Jun. 30, 2011
Intangible Assets and Goodwill [Abstract]
Intangible assets	As of June 30, 2011     As of December 31, 2010       Estimated           Accumulated                 Accumulated         (In millions)   Life     Cost     Amortization     Net     Cost     Amortization     Net     Out-licensed patents     12 years      $ 578.0      $ (370.9 )    $ 207.1      $ 578.0      $ (350.2 )    $ 227.8   Core developed technology     15-23 years       3,005.3       (1,723.4 )     1,281.9       3,005.3       (1,636.9 )     1,368.4   In process research and development     Up to 15 years upon commercialization       110.9       —       110.9       110.9       —       110.9   Trademarks and tradenames     Indefinite       64.0       —       64.0       64.0       —       64.0   In-licensed patents     Up to 14 years       17.5       (2.5 )     15.0       3.0       (1.3 )     1.7   Assembled workforce     4 years       2.1       (2.1 )     —       2.1       (2.1 )     —   Distribution rights     2 years       12.7       (12.7 )     —       12.7       (12.7 )     —                                                             Total intangible assets            $ 3,790.5      $ (2,111.6 )    $ 1,678.9      $ 3,776.0      $ (2,003.2 )    $ 1,772.8

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Fair Value Measurements (Tables)	6 Months Ended
Jun. 30, 2011
Fair Value Measurements [Abstract]
Summary of Assets and Liabilities Recorded at Fair Value	Significant       Balance as of     Quoted Prices in     Significant Other     Unobservable       June 30,     Active Markets     Observable Inputs     Inputs   (In millions)   2011     (Level 1)     (Level 2)     (Level 3)     Assets:                                 Cash equivalents    $ 440.6      $ —      $ 440.6      $ —   Marketable debt securities:                                 Corporate debt securities     475.1       —       475.1       —   Government securities     1,095.4       —       1,095.4       —   Mortgage and other asset backed securities     242.3       —       242.3       —   Strategic investments     2.0       2.0       —       —   Venture capital investments     20.6       —       —       20.6   Derivative contracts     0.5       —       0.5       —   Plan assets for deferred compensation     15.2       —       15.2       —                                     Total    $ 2,291.7      $ 2.0      $ 2,269.1      $ 20.6                                     Liabilities:                                 Derivative contracts    $ 24.9      $ —      $ 24.9      $ —   Acquisition-related contingent consideration     84.6       —       —       84.6                                     Total    $ 109.5      $ —      $ 24.9      $ 84.6                                                                                               Significant       Balance as of     Quoted Prices in     Significant Other     Unobservable       December 31,     Active Markets     Observable Inputs     Inputs   (In millions)   2010     (Level 1)     (Level 2)     (Level 3)     Assets:                                 Cash equivalents    $ 651.8      $ —      $ 651.8      $ —   Marketable debt securities:                                 Corporate debt securities     313.0       —       313.0       —   Government securities     785.3       —       785.3       —   Mortgage and other asset backed securities     92.9       —       92.9       —   Strategic investments     44.8       44.8       —       —   Venture capital investments     20.8       —       —       20.8   Derivative contracts     1.3       —       1.3       —   Plan assets for deferred compensation     13.0       —       13.0       —                                     Total    $ 1,922.9      $ 44.8      $ 1,857.3      $ 20.8                                     Liabilities:                                 Derivative contracts    $ 12.2      $ —      $ 12.2      $ —   Acquisition-related contingent consideration     81.2       —       —       81.2                                     Total    $ 93.4      $ —      $ 12.2      $ 81.2
Fair value of venture capital investments	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Beginning balance    $ 20.5      $ 20.8      $ 20.8      $ 21.9   Unrealized gains included in earnings     0.1       —       0.7       —   Unrealized losses included in earnings     (0.3 )     (0.1 )     (1.3 )     (1.6 ) Purchases     0.3       (0.3 )     0.4       0.1   Issuances     —       —       —       —   Settlements     —       —       —       —                                     Ending balance    $ 20.6      $ 20.4      $ 20.6      $ 20.4
Summary of fair and carrying value of debt instruments	As of June 30, 2011     As of December 31, 2010       Fair     Carrying     Fair     Carrying   (In millions)   Value     Value     Value     Value     Credit line from Dompé    $ 4.3      $ 4.3      $ 8.1      $ 8.0   Note payable to Fumedica     24.4       21.7       24.2       22.0   6.0% Senior Notes due 2013     482.9       449.8       485.5       449.8   6.875% Senior Notes due 2018     639.4       595.1       618.0       597.9                                     Total    $ 1,151.0      $ 1,070.9      $ 1,135.8      $ 1,077.7

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Financial Instruments (Tables)	6 Months Ended
Jun. 30, 2011
Financial Instruments [Abstract]
Marketable Securities including Strategic Investments	Gross     Gross             Fair     Unrealized     Unrealized     Amortized   As of June 30, 2011 (In millions)   Value     Gains     Losses     Cost     Available-for-sale                                 Corporate debt securities                                 Current    $ 122.3      $ 0.2      $ —      $ 122.1   Non-current     352.8       1.3       (0.2 )     351.7   Government securities                                 Current     504.7       0.3       —       504.4   Non-current     590.7       0.8       (0.1 )     590.0   Mortgage and other asset backed securities                                 Current     2.2       —       —       2.2   Non-current     240.1       0.5       (0.6 )     240.2                                     Total available-for-sale securities    $ 1,812.8      $ 3.1      $ (0.9 )    $ 1,810.6                                     Other Investments                                 Strategic investments, non-current    $ 2.0      $ 0.5      $ —      $ 1.5                                                                                   Gross     Gross             Fair     Unrealized     Unrealized     Amortized   As of December 31, 2010 (In millions)   Value     Gains     Losses     Cost     Available-for-sale                                 Corporate debt securities                                 Current    $ 93.2      $ 0.1      $ —      $ 93.1   Non-current     219.8       2.1       (0.5 )     218.2   Government securities                                 Current     352.8       0.2       —       352.6   Non-current     432.5       0.6       (0.6 )     432.5   Mortgage and other asset backed securities                                 Current     2.1       —       —       2.1   Non-current     90.8       0.5       (0.2 )     90.5                                     Total available-for-sale securities    $ 1,191.2      $ 3.5      $ (1.3 )    $ 1,189.0                                     Other Investments                                 Strategic investments, non-current    $ 44.8      $ 17.5      $ —      $ 27.3
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents	As of     As of       June 30,     December 31,   (In millions)   2011     2010     Commercial paper    $ 8.0      $ 4.0   Repurchase agreements     80.9       26.0   Short-term debt securities     351.7       621.8                     Total    $ 440.6      $ 651.8
Summary of Contractual Maturities: Available-for-Sale Securities	As of June 30, 2011     As of December 31, 2010       Estimated     Amortized     Estimated     Amortized   (In millions)   Fair Value     Cost     Fair Value     Cost     Due in one year or less    $ 629.2      $ 628.7      $ 448.1      $ 447.8   Due after one year through five years     1,040.9       1,039.0       664.1       662.4   Due after five years     142.7       142.9       79.0       78.8                                     Total    $ 1,812.8      $ 1,810.6      $ 1,191.2      $ 1,189.0
Proceeds from Marketable Securities, excluding Strategic Investments	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Proceeds from maturities and sales    $ 381.8      $ 973.2      $ 1,169.8      $ 2,002.5   Realized gains    $ 0.7      $ 7.4      $ 3.1      $ 13.1   Realized losses    $ (0.5 )    $ 1.1      $ (1.3 )    $ 1.8

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Derivative Instruments (Tables)	6 Months Ended
Jun. 30, 2011
Derivative Instruments [Abstract]
Foreign currency forward contracts that were entered into to hedge forecasted revenue	Notional Amount       As of     As of       June 30,     December 31,   Foreign Currency (In millions)   2011     2010     Euro    $ 537.1      $ 460.3   Canadian dollar     11.4       24.0   Swedish krona     4.8       9.9                     Total foreign currency forward contracts    $ 553.3      $ 494.2
Summary of Derivatives designated as Hedging Instruments	Fair Value         As of June 30, (In millions)   Balance Sheet Location   2011   Foreign Currency Contracts             Asset derivatives   Other current assets    $ —   Liability derivatives   Accrued expenses and other    $ 23.8                           Fair Value         As of December 31, (In millions)   Balance Sheet Location   2010   Foreign Currency Contracts             Asset derivatives   Other current assets    $ —   Liability derivatives   Accrued expenses and other    $ 11.0
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income	Amount       Amount             Recognized in       Reclassified from             Accumulated       Accumulated             Other       Other             Comprehensive       Comprehensive             Income (Loss)   Income   Income (Loss)   Income   Amount of     on Derivative   Statement   into Income   Statement   Gain/(Loss)     Gain/(Loss)   Location   Gain/(Loss)   Location   Recorded (In millions)   (Effective Portion)   (Effective Portion)   (Effective Portion)   (Ineffective Portion)   (Ineffective Portion)   For the Three Months Ended                                 June 30, 2011:                       Other income         Foreign currency contracts    $ (23.3 )   Revenue    $ (18.5 )   (expense)    $ (1.2 ) June 30, 2010:                       Other income         Foreign currency contracts    $ 62.1     Revenue    $ 19.7     (expense)    $ (0.6 )                                   For the Six Months Ended                                 June 30, 2011:                       Other income         Foreign currency contracts    $ (23.3 )   Revenue    $ (26.8 )   (expense)    $ (0.5 ) June 30, 2010:                       Other income         Foreign currency contracts    $ 62.1     Revenue    $ 19.9     (expense)    $ (0.5 )

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Comprehensive Income (Tables)	6 Months Ended
Jun. 30, 2011
Equity and Comprehensive Income [Abstract]
Activities reflected in comprehensive income, included within equity attributable to the shareholders of Biogen Idec, equity attributable to noncontrolling interests, and total shareholders' equity	For the Three Months     For the Three Months       Ended June 30, 2011     Ended June 30, 2010       Biogen Idec           Total     Biogen Idec           Total       Shareholders’     Noncontrolling     Shareholders’     Shareholders’     Noncontrolling     Shareholders’   (In millions)   Equity     Interests     Equity     Equity     Interests     Equity     Comprehensive income:                                                 Net income    $ 288.0      $ 16.0      $ 304.0      $ 293.4      $ 1.2      $ 294.6   Unrealized gains (losses) on securities available for sale, net of tax of  $0.6 and  $3.8     1.1       —       1.1       (6.5 )     —       (6.5 ) Unrealized gains (losses) on foreign currency forward contracts, net of tax of  $0.8 and  $3.6     6.3       —       6.3       26.5       —       26.5   Unrealized gains (losses) on pension benefit obligation, net of tax of  $0 and  $0     —       —       —       (0.2 )     —       (0.2 ) Currency translation adjustment     19.3       2.9       22.2       (71.8 )     (3.4 )     (75.2 )                                                   Comprehensive income (loss)    $ 314.7      $ 18.9      $ 333.6      $ 241.4      $ (2.2 )    $ 239.2                                                                                                             For the Six Months     For the Six Months       Ended June 30, 2011     Ended June 30, 2010       Biogen Idec           Total     Biogen Idec           Total       Shareholders’     Noncontrolling     Shareholders’     Shareholders’     Noncontrolling     Shareholders’   (In millions)   Equity     Interests     Equity     Equity     Interests     Equity     Comprehensive income:                                                 Net income    $ 582.4      $ 30.4      $ 612.8      $ 510.9      $ 3.8      $ 514.7   Unrealized gains (losses) on securities available for sale, net of tax of  $6.3 and  $5.5     (10.7 )     —       (10.7 )     (9.4 )     —       (9.4 ) Unrealized gains (losses) on foreign currency forward contracts, net of tax of  $1.2 and  $6.5     (11.1 )     —       (11.1 )     54.3       —       54.3   Unrealized gains (losses) on pension benefit obligation, net of tax of  $0 and  $0     —       —       —       (0.3 )     —       (0.3 ) Currency translation adjustment     69.1       5.7       74.8       (124.2 )     (6.0 )     (130.2 )                                                   Comprehensive income (loss)    $ 629.7      $ 36.1      $ 665.8      $ 431.3      $ (2.2 )    $ 429.1
Stockholders' Equity attributable to Noncontrolling interest- roll forward	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Noncontrolling interests, beginning of period    $ 70.1      $ 41.2      $ 52.9      $ 40.4   Net income attributable to noncontrolling interests     16.0       1.2       30.4       3.8   Translation adjustments     2.9       (3.4 )     5.7       (6.0 ) Distributions to noncontrolling interests     (9.9 )     —       (9.9 )     —   Capital contributions from noncontrolling interests     —       1.4       —       2.2                                   Noncontrolling interests, end of period    $ 79.1      $ 40.4      $ 79.1      $ 40.4

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Earnings per Share (Tables)	6 Months Ended
Jun. 30, 2011
Earnings per Share [Abstract]
Basic and diluted earnings per share	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Numerator:                                 Net income attributable to Biogen Idec Inc    $ 288.0      $ 293.4      $ 582.4      $ 510.9   Adjustment for net income allocable to preferred stock     —       (0.6 )     (0.5 )     (0.9 )                                 Net income used in calculating basic and diluted earnings per share    $ 288.0      $ 292.8      $ 581.9      $ 510.0                                   Denominator:                                 Weighted average number of common shares outstanding     242.4       259.9       241.9       265.0   Effect of dilutive securities:                                 Stock options and employee stock purchase plan     1.0       0.8       1.3       0.9   Time-vested restricted stock units     1.4       1.0       1.5       1.4   Market stock units     0.2       —       0.2       —   Performance-vested restricted stock units settled in shares     —       —       —       —                                   Dilutive potential common shares     2.6       1.8       3.0       2.3                                   Shares used in calculating diluted earnings per share     245.0       261.7       244.9       267.3
Anti dilutive securities excluded from computation of dilutive earnings per share	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Numerator:                                 Net income allocable to preferred stock    $ —      $ 0.6      $ 0.5      $ 0.9                                     Denominator:                                 Stock options     —       5.3       —       5.1   Time-vested restricted stock units     —       1.4       —       1.0   Market stock units     —       —       —       —   Performance-vested restricted stock units settled in shares     —       —       —       —   Convertible preferred stock     —       0.5       0.2       0.5                                     Total     —       7.2       0.2       6.6

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Share-Based Payments (Tables)	6 Months Ended
Jun. 30, 2011
Share-based Payments [Abstract]
Schedule of stock option and restricted stock unit grants	For the Six Months     Ended June 30,     2011   2010   Stock options     —       124,000   Market stock units(a)     373,000       334,000   Cash settled performance shares(b)     483,000       373,000   Time-vested restricted stock units(c)     1,303,000       1,700,000   Performance-vested restricted stock units(d)     1,000       4,000       (a) Market stock units (MSUs) granted for the six months ended June 30, 2010, represents the target number of shares eligible to be earned at the time of grant.   MSUs granted for the six months ended June 30, 2011, includes approximately 18,000 additional MSUs issued in 2011 based upon the attainment of performance criteria set for 2010 in relation to shares granted in 2010. The remainder of the MSUs granted in 2011 represent the target number of shares eligible to be earned at the time of grant. These grants were made in conjunction with the hiring of employees and our annual awards made in February.   (b) Cash settled performance shares (CSPSs) granted for the six months ended June 30, 2010, represents the target number of shares eligible to be earned at the time of grant.   CSPSs granted for the six months ended June 30, 2011, includes approximately 95,000 additional CSPSs issued in 2011 based upon the attainment of performance criteria set for 2010 in relation to shares granted in 2010. The remainder of the CSPSs granted in 2011 represent the target number of shares eligible to be earned at the time of grant. These grants were made in conjunction with the hiring of employees and our annual awards made in February.   (c) Time-vested restricted stock units (RSUs) granted for the six months ended June 30, 2011, includes approximately 1.2 million RSUs granted in connection with our annual awards made in February 2011, and 135,000 RSUs granted in conjunction with new hires and grants made to our Board of Directors.   (d) Performance-vested restricted stock units (PVRSUs) granted for the six months ended June 30, 2010, represents the target number of shares eligible to be earned at the time of grant; approximately 1,000 additional PVRSUs were issued in 2011 based upon the attainment of performance criteria set for 2010 in relation to shares granted in 2010.
Share-based compensation expense included in consolidated statements of income	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Research and development    $ 13.9      $ 15.4      $ 32.2      $ 32.1   Selling, general and administrative     22.2       34.0       42.8       70.2   Restructuring charges     —       —       (0.6 )     —                                     Subtotal     36.1       49.4       74.4       102.3   Capitalized share-based compensation costs     (1.0 )     (0.7 )     (2.0 )     (1.6 )                                   Share-based compensation expense included in total cost and expenses     35.1       48.7       72.4       100.7   Income tax effect     (10.9 )     (15.7 )     (23.0 )     (32.4 )                                   Share-based compensation expense included in net income attributable to Biogen Idec Inc    $ 24.2      $ 33.0      $ 49.4      $ 68.3
Summary of share-based compensation expense associated with different programs	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Stock options    $ 1.6      $ 9.3      $ 2.7      $ 20.1   Market stock units     4.3       1.9       7.7       5.5   Time-vested restricted stock units     19.1       32.3       46.2       65.8   Performance-vested restricted stock units settled in shares     0.3       1.3       0.7       3.7   Cash settled performance shares     10.7       4.2       15.5       5.2   Employee stock purchase plan     0.1       0.4       1.6       2.0                                     Subtotal     36.1       49.4       74.4       102.3   Capitalized share-based compensation costs     (1.0 )     (0.7 )     (2.0 )     (1.6 )                                   Share-based compensation expense included in total cost and expenses    $ 35.1      $ 48.7      $ 72.4      $ 100.7

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Income Taxes (Tables)	6 Months Ended
Jun. 30, 2011
Income Taxes [Abstract]
Reconciliation between the U.S. federal statutory tax rate and effective tax rate	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,       2011     2010     2011     2010     Statutory rate     35.0 %     35.0 %     35.0 %     35.0 % State taxes     0.4       1.9       1.4       1.9   Taxes on foreign earnings     (7.3 )     (10.3 )     (6.3 )     (10.1 ) Credits and net operating loss utilization     (4.1 )     (1.6 )     (3.1 )     (1.6 ) Purchased intangible assets     1.4       1.5       1.4       1.5   IPR&D     —       0.8       —       0.8   Permanent items     (1.1 )     (1.7 )     (1.2 )     (1.7 ) Other     (0.5 )     0.2       (1.5 )     (0.1 )                                   Effective tax rate     23.8 %     25.8 %     25.7 %     25.7 %

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Other Consolidated Financial Statement Detail (Tables)	6 Months Ended
Jun. 30, 2011
Business and Collaborations [Abstract]
Other income (expense), net	For the Three Months     For the Six Months       Ended June 30,     Ended June 30,   (In millions)   2011     2010     2011     2010     Interest income    $ 4.3      $ 6.7      $ 8.0      $ 15.6   Interest expense     (8.4 )     (9.0 )     (17.6 )     (17.3 ) Impairments of investments     (5.5 )     (1.2 )     (6.8 )     (17.0 ) Foreign exchange gains (losses), net     (0.6 )     (0.7 )     (1.0 )     0.3   Gain (loss) on sales of investments, net     0.2       6.3       15.5       11.3   Other, net     (1.7 )     (1.1 )     0.1       (0.3 )                                   Total other income (expense), net    $ (11.7 )    $ 1.0      $ (1.8 )    $ (7.4 )
Other current assets	As of     As of       June 30,     December 31,   (In millions)   2011     2010     Deferred tax assets    $ 66.9      $ 112.2   Prepaid taxes     33.1       31.4   Receivable from collaborations     8.8       7.3   Interest receivable     6.1       4.9   Other prepaid expenses     60.1       47.9   Other     26.8       12.1                     Total other current assets    $ 201.8      $ 215.8
Accrued Expenses and Other	As of     As of       June 30,     December 31,   (In millions)   2011     2010     Employee compensation and benefits    $ 134.3      $ 159.7   Revenue-related rebates     112.4       105.3   Restructuring charges     8.9       66.4   Royalties and licensing fees     40.0       45.1   Deferred revenue     59.1       41.3   Collaboration expenses     57.1       31.6   Clinical development expenses     33.7       24.4   Interest payable     21.6       21.6   Construction in progress acrual     13.3       16.4   Current portion of contingent consideration     4.9       11.9   Derivative liability     24.9       12.2   Other     131.1       130.0                     Total accrued expenses and other    $ 641.3      $ 665.9

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Investments in Variable Interest Entities (Tables)	6 Months Ended
Jun. 30, 2011
Investments in Variable Interest Entities [Abstract]
Total joint venture assets and Liabilities	As of   As of     June 30,   December 31, (In millions)   2011   2010   Assets    $ 193.6      $ 159.2   Liabilities    $ 74.6      $ 63.3

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Business (Details) (USD  $) In Millions, unless otherwise specified	6 Months Ended	0 Months Ended
Jun. 30, 2011	Jul. 14, 2011 New Cambridge Leases [Member]	Jul. 20, 2011 Acorda [Member]
Variable Interest Entity [Line Items]
Area of office building under lease		500,000
Milestone payment payable to Acorda Therapeutics, Inc. (Acorda)			$ 25
Business (Textuals) [Abstract]
Interest in subsidiary (less than given percentage)	100.00%

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Acquisitions (Details) (USD  $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended			3 Months Ended
Jun. 30, 2011	Jun. 30, 2011	Jun. 30, 2011 Accrued expenses and other [Member] Panima Pharmaceuticals AG [Member]	Jun. 30, 2011 Other long-term liabilities [Member] Panima Pharmaceuticals AG [Member]	Jun. 30, 2011 Panima Pharmaceuticals AG [Member]	Dec. 17, 2010 Panima Pharmaceuticals AG [Member]	Mar. 31, 2011 Biogen Idec Hemophilia [Member]
Acquisitions (Textuals) [Abstract]
Stock acquired in acquisition						100.00%
Cash portion of consideration						$ 32.5
Maximum contingent cash payments						395
Contingent consideration			4.9	79.7	84.6	81.2
Payment to former shareholders of Syntonix Pharmaceuticals							40
Contingent consideration expense	$ 2.2	$ 3.4

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Restructuring (Details) (USD  $)	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended
Dec. 31, 2010	Jun. 30, 2011	Jun. 30, 2011 Workforce reduction [Member]	Jun. 30, 2011 Workforce reduction [Member]	Jun. 30, 2011 Facility consolidation [Member]	Jun. 30, 2011 Restructuring depreciation [Member]	Jun. 30, 2011 Restructuring depreciation [Member]
Restructuring liability
Restructuring reserves as of December 31, 2010		$ 66,400,000		$ 60,600,000	$ 5,800,000
Expense		14,500,000		12,100,000	2,400,000
(Payments) receipts, net		(75,800,000)		(73,800,000)	(2,000,000)
Adjustments to previous estimates, net		(1,600,000)		(1,600,000)
Other adjustments		5,400,000		8,600,000	(3,200,000)
Restructuring reserves as of June 30, 2011	66,400,000	8,900,000	5,900,000	5,900,000	3,000,000
Restructuring (Textuals) [Abstract]
Total restructuring charges	75,200,000	16,587,000	2,700,000	10,500,000		1,500,000	6,000,000
Restructuring (Textuals) [Abstract]
Headcount reduction due to consolidation of sites		13.00%
Approximate restructuring cost due to reduction in workforce and closing and consolidation of certain leased and owned facilities		100,000,000
Restructuring charges offset by net adjustments		$ 4,400,000

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Revenue Recognition (Details) (USD  $) In Millions	6 Months Ended
Jun. 30, 2011
Analysis of the amount of, and change in, product revenue reserves
Period balance related to product revenue reserves	$ 142
Current provisions relating to sales in current year	228.6
Adjustments relating to prior years	(8.4)
Payments/returns applied against product revenue reserves relating to current year	(135)
Payments/returns relating to sales in prior year	(76.4)
Period balance related to product revenue reserves	150.8
Discounts [Member]
Analysis of the amount of, and change in, product revenue reserves
Period balance related to product revenue reserves	13.9
Current provisions relating to sales in current year	47.2
Payments/returns applied against product revenue reserves relating to current year	(33.3)
Payments/returns relating to sales in prior year	(13)
Period balance related to product revenue reserves	14.8
Contractual Adjustments [Member]
Analysis of the amount of, and change in, product revenue reserves
Period balance related to product revenue reserves	107
Current provisions relating to sales in current year	174.6
Adjustments relating to prior years	(8.4)
Payments/returns applied against product revenue reserves relating to current year	(101.4)
Payments/returns relating to sales in prior year	(58.5)
Period balance related to product revenue reserves	113.3
Returns [Member]
Analysis of the amount of, and change in, product revenue reserves
Period balance related to product revenue reserves	21.1
Current provisions relating to sales in current year	6.8
Payments/returns applied against product revenue reserves relating to current year	(0.3)
Payments/returns relating to sales in prior year	(4.9)
Period balance related to product revenue reserves	$ 22.7

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Revenue Recognition (Details 1) (USD  $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Summary of total product revenue reserves included in consolidated balance sheets
Reduction of accounts receivable	$ 38.4	$ 36.7
Current liability	112.4	105.3
Period balance related to product revenue reserves	$ 150.8	$ 142

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Accounts Receivable (Details)	1 Months Ended	6 Months Ended
May 31, 2011 EUR ( €)	Jun. 30, 2011 USD ( $)	Dec. 31, 2010 USD ( $)	Jun. 30, 2011 Accounts Receivable Net [Member] Italy [Member] USD ( $)	Dec. 31, 2010 Accounts Receivable Net [Member] Italy [Member] USD ( $)	Jun. 30, 2011 Accounts Receivable Net [Member] Spain [Member] USD ( $)	Dec. 31, 2010 Accounts Receivable Net [Member] Spain [Member] USD ( $)	Jun. 30, 2011 Accounts Receivable Net [Member] Portugal [Member] USD ( $)	Dec. 31, 2010 Accounts Receivable Net [Member] Portugal [Member] USD ( $)	Jun. 30, 2011 Accounts Receivable Net [Member] Greece [Member] USD ( $)	Dec. 31, 2010 Accounts Receivable Net [Member] Greece [Member] USD ( $)	Jun. 30, 2011 Investments And Other Assets [Member] Italy [Member] USD ( $)	Dec. 31, 2010 Investments And Other Assets [Member] Italy [Member] USD ( $)	Jun. 30, 2011 Investments And Other Assets [Member] Spain [Member] USD ( $)	Dec. 31, 2010 Investments And Other Assets [Member] Spain [Member] USD ( $)	Jun. 30, 2011 Investments And Other Assets [Member] Portugal [Member] USD ( $)	Dec. 31, 2010 Investments And Other Assets [Member] Portugal [Member] USD ( $)	Jun. 30, 2011 Investments And Other Assets [Member] Greece [Member] USD ( $)	Dec. 31, 2010 Investments And Other Assets [Member] Greece [Member] USD ( $)	Jun. 30, 2011 Italy [Member] USD ( $)	Dec. 31, 2010 Italy [Member] USD ( $)	Jun. 30, 2011 Spain [Member] USD ( $)	Dec. 31, 2010 Spain [Member] USD ( $)	Jun. 30, 2011 Portugal [Member] USD ( $)	Dec. 31, 2010 Portugal [Member] USD ( $)	Jun. 30, 2011 Greece [Member] USD ( $)	Dec. 31, 2010 Greece [Member] USD ( $)
Account receivable balance from different countries
Accounts Receivable, Total				$ 116,500,000	$ 103,200,000	$ 81,700,000	$ 70,800,000	$ 23,900,000	$ 17,800,000	$ 10,700,000	$ 3,900,000	$ 11,300,000	$ 14,800,000	$ 36,700,000	$ 29,800,000	$ 7,400,000	$ 5,500,000	$ 0	$ 0	$ 127,800,000	$ 118,000,000	$ 118,400,000	$ 100,600,000	$ 31,300,000	$ 23,300,000	$ 10,700,000	$ 3,900,000
Accounts Receivable (Textuals) [Abstract]
Payment terms of accounts receivable arising from product sales		Between 30 and 90 days
Rescue package approved term	3 years
Rescue package approved amount	78,000,000,000
Accounts receivable outstanding for greater than one year		$ 53,500,000	$ 45,000,000

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Inventory (Details) (USD  $)	Jun. 30, 2011	Dec. 31, 2010
Components of inventories
Raw materials	$ 59,800,000	$ 59,000,000
Work in process	165,600,000	142,200,000
Finished goods	82,900,000	87,900,000
Total inventory	$ 308,254,000	$ 289,066,000

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Intangible Assets and Goodwill (Details) (USD  $)	3 Months Ended	6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended			3 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 In-Process Research and Development [Member]	Dec. 31, 2010 In-Process Research and Development [Member]	Jun. 30, 2011 Trademarks and tradenames [Member]	Dec. 31, 2010 Trademarks and tradenames [Member]	Jun. 30, 2011 Out-licensed patents [Member]	Dec. 31, 2010 Out-licensed patents [Member]	Jun. 30, 2011 Core developed technology [Member]	Dec. 31, 2010 Core developed technology [Member]	Jun. 30, 2011 In-licensed patents [Member]	Dec. 31, 2010 In-licensed patents [Member]	Jun. 30, 2011 Assembled workforce [Member]	Dec. 31, 2010 Assembled workforce [Member]	Jun. 30, 2011 Distribution rights [Member]	Dec. 31, 2010 Distribution rights [Member]	Jun. 30, 2011 Avonex [Member]	Mar. 31, 2011 VP1 Protein [Member]	Jun. 30, 2011 VP1 Protein [Member]
Indefinite-lived Intangible Assets by Major Class [Line Items]
Estimated Life						Up to 15 years upon commercialization		Indefinite
Cost and Net						$ 110,900,000	$ 110,900,000	$ 64,000,000	$ 64,000,000
Intangible assets
Estimated Life (In Years)										12				14		4		2
Estimated Life, Minimum (In Years)												15
Estimated Life, Maximum (In Years)												23
Cost										578,000,000	578,000,000	3,005,300,000	3,005,300,000	17,500,000	3,000,000	2,100,000	2,100,000	12,700,000	12,700,000
Total intangible assets, gross	3,790,500,000		3,790,500,000		3,776,000,000																	14,500,000
Accumulated Amortization	(2,111,600,000)		(2,111,600,000)		(2,003,200,000)					(370,900,000)	(350,200,000)	(1,723,400,000)	(1,636,900,000)	(2,500,000)	(1,300,000)	(2,100,000)	(2,100,000)	(12,700,000)	(12,700,000)
Net										207,100,000	227,800,000	1,281,900,000	1,368,400,000	15,000,000	1,700,000	0	0	0	0	1,268,800,000
Intangible assets, net	1,678,867,000		1,678,867,000		1,772,826,000
Intangible Assets and Goodwill Additional (Textuals) [Abstract]
Net book value of the core technology intangible asset related to our AVONEX product										207,100,000	227,800,000	1,281,900,000	1,368,400,000	15,000,000	1,700,000	0	0	0	0	1,268,800,000
Expected usage-based royalties to be earned through 2016 from license agreement																					47,100,000
Intangible asset recognized to reflect the total of upfront and other time-based milestone payments expected to be made	3,790,500,000		3,790,500,000		3,776,000,000																	14,500,000
Intangible Assets and Goodwill (Textuals) [Abstract]
Amortization of acquired intangible assets	55,100,000	53,100,000	108,400,000	102,000,000
Amortization of intangible assets for each of next five years, Minimum			180,000,000
Amortization of intangible assets for each of next five years, Maximum			220,000,000
Accumulated impairment loss on goodwill	$ 0		$ 0

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Fair Value Measurements (Details) (USD  $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Liabilities:
Derivative contracts	$ 24,900,000		$ 24,900,000		$ 12,200,000
Investments and Acquisitions at Fair Value
Net cash outflow to determine valuations	395,000,000		395,000,000		395,000,000
Discount rate used for net cash outflow projections for fair value measurement	5.70%		5.70%		6.10%
Fair value adjustment of contingent consideration	2,200,000		3,400,000
Fair value of venture capital investments
Beginning balance, January 1	20,500,000	20,800,000	20,800,000	21,900,000
Unrealized gains included in earnings	100,000		700,000
Unrealized losses included in earnings	(300,000)	(100,000)	(1,300,000)	(1,600,000)
Purchases	300,000	(300,000)	400,000	100,000
Issuances	0		0
Settlements	0		0
Ending balance, June 30	20,600,000	20,400,000	20,600,000	20,400,000
Summary of fair and carrying value of debt instruments
Notes payable, Carrying Value	1,070,900,000		1,070,900,000		1,077,700,000
Total	1,551,000,000		1,551,000,000		1,135,800,000
Fair Value Measurements (Textuals) [Abstract]
Venture capital investments as a percentage of our total assets	approximately 0.2% of total assets		approximately 0.2% of total assets		approximately 0.3% of total assets
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents, Fair Value Disclosure	0		0		0
Derivative contracts	0		0		0
Plan assets for deferred compensation	0		0		0
Total	2,000,000		2,000,000		44,800,000
Liabilities:
Derivative contracts	0		0		0
Acquisition-related Contingent consideration	0		0		0
Total	0		0		0
Fair Value Measurements (Additional Textuals) [Abstract]
Acquisition-related Contingent consideration	0		0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Strategic investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	2,000,000		2,000,000		44,800,000
Quoted Prices in Active Markets, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents, Fair Value Disclosure	440,600,000		440,600,000		651,800,000
Derivative contracts	500,000		500,000		1,300,000
Plan assets for deferred compensation	15,200,000		15,200,000		13,000,000
Total	2,269,100,000		2,269,100,000		1,857,300,000
Liabilities:
Derivative contracts	24,900,000		24,900,000		12,200,000
Acquisition-related Contingent consideration	0		0		0
Total	24,900,000		24,900,000		12,200,000
Fair Value Measurements (Additional Textuals) [Abstract]
Acquisition-related Contingent consideration	0		0		0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	475,100,000		475,100,000		313,000,000
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	1,095,400,000		1,095,400,000		785,300,000
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	242,300,000		242,300,000		92,900,000
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member] | Strategic investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Unobservable Inputs (Level 3)
Liabilities:
Acquisition-related Contingent consideration	84,600,000		84,600,000		81,200,000
Fair Value Measurements (Additional Textuals) [Abstract]
Acquisition-related Contingent consideration	84,600,000		84,600,000		81,200,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents, Fair Value Disclosure	0		0		0
Derivative contracts	0		0		0
Plan assets for deferred compensation	0		0		0
Total	20,600,000		20,600,000		20,800,000
Liabilities:
Derivative contracts	0		0		0
Acquisition-related Contingent consideration	84,600,000		84,600,000		81,200,000
Total	84,600,000		84,600,000		81,200,000
Fair Value Measurements (Additional Textuals) [Abstract]
Acquisition-related Contingent consideration	84,600,000		84,600,000		81,200,000
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member] | Strategic investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	0		0		0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	20,600,000		20,600,000		20,800,000
Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents, Fair Value Disclosure	440,600,000		440,600,000		651,800,000
Derivative contracts	500,000		500,000		1,300,000
Plan assets for deferred compensation	15,200,000		15,200,000		13,000,000
Total	2,291,700,000		2,291,700,000		1,922,900,000
Liabilities:
Derivative contracts	24,900,000		24,900,000		12,200,000
Acquisition-related Contingent consideration	84,600,000		84,600,000		81,200,000
Total	109,500,000		109,500,000		93,400,000
Fair Value Measurements (Additional Textuals) [Abstract]
Acquisition-related Contingent consideration	84,600,000		84,600,000		81,200,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	475,100,000		475,100,000		313,000,000
Fair Value, Measurements, Recurring [Member] | Government securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	1,095,400,000		1,095,400,000		785,300,000
Fair Value, Measurements, Recurring [Member] | Mortgage and other asset backed securities [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	242,300,000		242,300,000		92,900,000
Fair Value, Measurements, Recurring [Member] | Strategic investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	2,000,000		2,000,000		44,800,000
Fair Value, Measurements, Recurring [Member] | Venture capital investments [Member]
Assets:
Fair Value, Measured on Recurring Basis, Investments	20,600,000		20,600,000		20,800,000
Dompe [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	4,300,000		4,300,000		8,100,000
Notes payable, Carrying Value	4,300,000		4,300,000		8,000,000
Notes Payable to Fumedica [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	24,400,000		24,400,000		24,200,000
Notes payable, Carrying Value	21,700,000		21,700,000		22,000,000
6.0% Senior Notes due 2013 [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	482,900,000		482,900,000		485,500,000
Notes payable, Carrying Value	449,800,000		449,800,000		449,800,000
6.875% Senior Notes due 2018 [Member]
Summary of fair and carrying value of debt instruments
Notes payable, Fair Value	639,400,000		639,400,000		618,000,000
Notes payable, Carrying Value	$ 595,100,000		$ 595,100,000		$ 597,900,000

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Financial Instruments (Details) (USD  $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Marketable Securities including Strategic Investments
Fair Value	$ 1,812,800,000		$ 1,191,200,000		$ 1,812,800,000
Gross Unrealized Gains	3,100,000		3,500,000		3,100,000
Gross Unrealized Losses	(900,000)		(1,300,000)		(900,000)
Amortized Cost	1,810,600,000		1,189,000,000		1,810,600,000
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	440,600,000		651,800,000		440,600,000
Summary of Contractual Maturities: Available-for-Sale Securities
Due in one year or less, Estimated Fair Value	629,200,000		448,100,000		629,200,000
Due in one year or less, Amortized Cost	628,700,000		447,800,000		628,700,000
Due after one year through five years, Estimated Fair Value	1,040,900,000		664,100,000		1,040,900,000
Due after one year through five years, Amortized Cost	1,039,000,000		662,400,000		1,039,000,000
Due after five years, Estimated Fair Value	142,700,000		79,000,000		142,700,000
Due after five years, Amortized Cost	142,900,000		78,800,000		142,900,000
Total available-for-sale securities, Fair Value	1,812,800,000		1,191,200,000		1,812,800,000
Total available-for-sale securities, Amortized Cost	1,810,600,000		1,189,000,000		1,810,600,000
Proceeds from Marketable Securities, excluding Strategic Investments
Proceeds from maturities and sales	381,800,000			973,200,000	1,169,836,000	2,002,543,000
Realized gains	700,000			7,400,000	3,100,000	13,100,000
Realized losses	(500,000)			1,100,000	(1,300,000)	1,800,000
Financial Instruments (Textuals) [Abstract]
Federal Deposit Insurance Corporation guaranteed senior notes issued under the Temporary Liquidity Guarantee Program	247,000,000		163,500,000		247,000,000
Original maturities of commercial paper and short-term debt securities					less than 90 days
Average maturity of marketable securities, months			11 months		13 months
Gain (loss) from sale of strategic equity investments		13,800,000
Impairment charges for publicly - held strategic investments, investments in venture capital funds and investments in privately - held companies	5,500,000			1,200,000	6,800,000	17,000,000
Corporate debt securities Current [Member]
Marketable Securities including Strategic Investments
Fair Value	122,300,000		93,200,000		122,300,000
Gross Unrealized Gains	200,000		100,000		200,000
Gross Unrealized Losses	0		0		0
Amortized Cost	122,100,000		93,100,000		122,100,000
Summary of Contractual Maturities: Available-for-Sale Securities
Total available-for-sale securities, Fair Value	122,300,000		93,200,000		122,300,000
Corporate debt securities Non-current [Member]
Marketable Securities including Strategic Investments
Fair Value	352,800,000		219,800,000		352,800,000
Gross Unrealized Gains	1,300,000		2,100,000		1,300,000
Gross Unrealized Losses	(200,000)		(500,000)		(200,000)
Amortized Cost	351,700,000		218,200,000		351,700,000
Summary of Contractual Maturities: Available-for-Sale Securities
Total available-for-sale securities, Fair Value	352,800,000		219,800,000		352,800,000
Government securities Current [Member]
Marketable Securities including Strategic Investments
Fair Value	504,700,000		352,800,000		504,700,000
Gross Unrealized Gains	300,000		200,000		300,000
Gross Unrealized Losses	0		0		0
Amortized Cost	504,400,000		352,600,000		504,400,000
Summary of Contractual Maturities: Available-for-Sale Securities
Total available-for-sale securities, Fair Value	504,700,000		352,800,000		504,700,000
Government securities Non-current [Member]
Marketable Securities including Strategic Investments
Fair Value	590,700,000		432,500,000		590,700,000
Gross Unrealized Gains	800,000		600,000		800,000
Gross Unrealized Losses	(100,000)		(600,000)		(100,000)
Amortized Cost	590,000,000		432,500,000		590,000,000
Summary of Contractual Maturities: Available-for-Sale Securities
Total available-for-sale securities, Fair Value	590,700,000		432,500,000		590,700,000
Mortgage and other asset backed securities Current [Member]
Marketable Securities including Strategic Investments
Fair Value	2,200,000		2,100,000		2,200,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Amortized Cost	2,200,000		2,100,000		2,200,000
Summary of Contractual Maturities: Available-for-Sale Securities
Total available-for-sale securities, Fair Value	2,200,000		2,100,000		2,200,000
Mortgage and other asset backed securities Non-current [Member]
Marketable Securities including Strategic Investments
Fair Value	240,100,000		90,800,000		240,100,000
Gross Unrealized Gains	500,000		500,000		500,000
Gross Unrealized Losses	(600,000)		(200,000)		(600,000)
Amortized Cost	240,200,000		90,500,000		240,200,000
Summary of Contractual Maturities: Available-for-Sale Securities
Total available-for-sale securities, Fair Value	240,100,000		90,800,000		240,100,000
Other strategic investments Non-current [Member]
Marketable Securities including Strategic Investments
Gross Unrealized Gains	500,000		17,500,000		500,000
Gross Unrealized Losses	0		0		0
Amortized Cost	1,500,000		27,300,000		1,500,000
Strategic investments, non-current Fair Value	2,000,000		44,800,000		2,000,000
Commercial Paper [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	8,000,000		4,000,000		8,000,000
Repurchase Agreements [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	80,900,000		26,000,000		80,900,000
Debt Securities [Member]
Summary of financial assets with original maturities of less than 90 days included within cash and cash equivalents
Cash equivalents	$ 351,700,000		$ 621,800,000		$ 351,700,000

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Derivative Instruments (Details) (USD  $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Foreign currency forward contracts that were entered into to hedge forecasted revenue
Total foreign currency forward contracts	$ 553.3		$ 553.3		$ 494.2
Derivative Instruments (Textuals) [Abstract]
Lower Range of Durations of foreign currency forward contracts	1 month		1 month		1 month
Higher Range of Durations of foreign currency forward contracts	12 months		12 months		12 months
Gain/Loss on fair value of Foreign currency forward contracts	23.8		23.8		11
Expected Settlement time for contracts, in months	12 months		12 months
Gains and losses in earnings of Foreign currency forward contracts due to hedge ineffectiveness	(1.2)	(0.6)	(0.5)	(0.5)
Gains/(Losses) in product revenue for the settlement of certain effective cash flow hedge instruments	(18.5)	19.7	(26.8)	19.9
Aggregate notional amount of outstanding foreign currency contracts	243.2		243.2
Fair Value of outstanding foreign currency contracts	0.7		0.7
Net losses of other income (expense) related to foreign currency forward contracts	0.6	7.9	4.3	13.1
Euro [Member]
Foreign currency forward contracts that were entered into to hedge forecasted revenue
Total foreign currency forward contracts	537.1		537.1		460.3
Canadian Dollar [Member]
Foreign currency forward contracts that were entered into to hedge forecasted revenue
Total foreign currency forward contracts	11.4		11.4		24
Swedish krona [Member]
Foreign currency forward contracts that were entered into to hedge forecasted revenue
Total foreign currency forward contracts	4.8		4.8		9.9
Foreign Exchange Contract [Member] | Other Current Assets [Member] | Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Asset, Fair Value	0		0		0
Foreign Exchange Contract [Member] | Accrued Expenses and Other Liabilities [Member] | Designated as Hedging Instrument [Member]
Summary of Derivatives designated as Hedging Instruments
Derivative Liability, Fair Value	$ 23.8		$ 23.8		$ 11

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Derivative Instruments (Details 1) (USD  $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income
Amount Reclassified from Accumulated Other Comprehensive Income into Income Gain/(Loss) (Effective Portion )	$ (18.5)	$ 19.7	$ (26.8)	$ 19.9
Foreign Exchange Contract [Member] | Revenue [Member]
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income
Amount Recognized in Accumulated Other Comprehensive Income on Derivative Gain/(Loss) (Effective Portion )	(23.3)	62.1	(23.3)	62.1
Amount Reclassified from Accumulated Other Comprehensive Income into Income Gain/(Loss) (Effective Portion )	(18.5)	19.7	(26.8)	19.9
Foreign Exchange Contract [Member] | Other income (expense) [Member]
Summary of the effect of derivatives designated as hedging instruments on the consolidated statements of income
Amount of Gain/(Loss) Recorded (Ineffective Portion )	$ (1.2)	$ (0.6)	$ (0.5)	$ (0.5)

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Property, Plant and Equipment (Details) (USD  $) In Millions, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010	Oct. 31, 2010 San Diego Facility [Member]	Jun. 30, 2011 San Diego Facility [Member]	Dec. 31, 2010 San Diego Facility [Member]	Jun. 30, 2011 Hillerod, Denmark Facility [Member]	Jun. 30, 2011 Hillerod, Denmark Facility [Member]	Dec. 31, 2010 Hillerod, Denmark Facility [Member]	Jul. 14, 2011 New Cambridge Leases [Member]
Property, Plant and Equipment (Textuals) [Abstract]
Accumulated depreciation on property, plant and equipment	$ 771.1	$ 767.2
Net proceeds from sale of San Diego facility			127
Lease transaction period in months				15 months
Remaining obligation of financing arrangements				124.5	125.9
Construction in progress balance related to biologic manufacturing facility						494.2	494.2	440.2
Interest cost capitalization related to large-scale biologic manufacturing facility						7.2	14.4
Area of office building under lease									500,000
Future minimum rental commitments under non- cancelable operating leases									$ 340
Operating leases term									15 years

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Equity (Details) (USD  $) In Millions, except Share data	1 Months Ended		3 Months Ended	6 Months Ended	1 Months Ended
Mar. 31, 2011	Feb. 28, 2011	Jun. 30, 2011	Jun. 30, 2011 Common Stock [Member]	Jun. 30, 2010 Common Stock [Member]	Jun. 30, 2011 Common Stock [Member]	Jun. 30, 2010 Common Stock [Member]	Mar. 31, 2011 Series A Preferred Stock [Member]
Equity (Textuals) [Abstract]
Repurchase of common stock, shares				2,200,000	20,800,000	5,000,000	31,300,000
Repurchase of common stock, value				$ 191.3	$ 1,000	$ 386.6	$ 1,600
Series A Preferred Stock shares Redeemed								8,221
Total number of common stock shares issued on on conversion of Series A Preferred Stock	493,260
Preferred Stock, shares issued			0
Preferred Stock, shares outstanding			0
Common stock shares authorized for repurchase		20,000,000

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Comprehensive Income (Details) (USD  $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2011	Dec. 31, 2010	Mar. 31, 2010	Dec. 31, 2009
Comprehensive income:
Net income	$ 304,049,000	$ 294,649,000	$ 612,814,000	$ 514,642,000
Unrealized gains (losses) on securities available for sale, net of tax of  $0.6 and  $3.8 for the three months ended and  $6.3 and  $5.5 for the six moths ended for June 2011 and 2010 respectively	1,100,000	(6,500,000)	(10,700,000)	(9,400,000)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of  $0.8 and  $3.6 for the three months ended and  $1.2 and  $6.5 for the six moths ended for June 2011 and 2010 respectively	6,300,000	26,500,000	(11,100,000)	54,300,000
Unrealized gains (losses) on pension benefit obligation, net of tax of  $0 and  $0 for the three months ended and  $0 and  $0 for the six moths ended for June 2011 and 2010 respectively		(200,000)		(300,000)
Currency translation adjustment	22,200,000	(75,200,000)	74,800,000	(130,200,000)
Comprehensive income (loss)	333,600,000	239,200,000	665,800,000	429,100,000
Stockholders' Equity attributable to Noncontrolling interest- roll forward
Net income attributable to noncontrolling interests	16,015,000	1,211,000	30,450,000	3,762,000
Translation adjustments	2,900,000	(3,400,000)	5,700,000	(6,000,000)
Distributions to noncontrolling interests	(9,900,000)		(9,900,000)
Capital contributions from noncontrolling interest		1,400,000		2,200,000
Noncontrolling interests, end of period	79,144,000	40,400,000	79,144,000	40,400,000	70,100,000	52,937,000	41,200,000	40,400,000
Biogen Idec Shareholders' Equity [Member]
Comprehensive income:
Net income	288,000,000	293,400,000	582,400,000	510,900,000
Unrealized gains (losses) on securities available for sale, net of tax of  $0.6 and  $3.8 for the three months ended and  $6.3 and  $5.5 for the six moths ended for June 2011 and 2010 respectively	1,100,000	(6,500,000)	(10,700,000)	(9,400,000)
Unrealized gains (losses) on foreign currency forward contracts, net of tax of  $0.8 and  $3.6 for the three months ended and  $1.2 and  $6.5 for the six moths ended for June 2011 and 2010 respectively	6,300,000	26,500,000	(11,100,000)	54,300,000
Unrealized gains (losses) on pension benefit obligation, net of tax of  $0 and  $0 for the three months ended and  $0 and  $0 for the six moths ended for June 2011 and 2010 respectively		(200,000)		(300,000)
Currency translation adjustment	19,300,000	(71,800,000)	69,100,000	(124,200,000)
Comprehensive income (loss)	314,700,000	241,400,000	629,700,000	431,300,000
Noncontrolling Interests [Member]
Comprehensive income:
Net income	16,000,000	1,200,000	30,400,000	3,800,000
Currency translation adjustment	2,900,000	(3,400,000)	5,700,000	(6,000,000)
Comprehensive income (loss)	$ 18,900,000	$ (2,200,000)	$ 36,100,000	$ (2,200,000)

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Comprehensive Income (Details) (Textual) (USD  $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Comprehensive Income (Textuals) [Abstract]
Net unrealized gains (losses) on securities available for sale, tax	$ 0.6	$ 3.8	$ 6.3	$ 5.5
Net unrealized gains (losses) on foreign currency forward contracts, tax	0.8	3.6	1.2	6.5
Net unrealized gains (losses) on pension benefit obligation, tax	$ 0	$ 0	$ 0	$ 0

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Earnings per Share (Details) (USD  $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Numerator:
Net income attributable to Biogen Idec Inc	$ 288,034,000	$ 293,438,000	$ 582,364,000	$ 510,880,000
Adjustment for net income allocable to preferred stock	0	(600,000)	(500,000)	(900,000)
Net income used in calculating basic and diluted earnings per share	288,000,000	292,800,000	581,900,000	510,000,000
Denominator:
Weighted average number of common shares outstanding	242,375,000	259,938,000	241,932,000	265,018,000
Effect of dilutive securities:
Stock options and employee stock purchase plan	0			0
Dilutive potential common shares	2,600,000	1,800,000	3,000,000	2,300,000
Shares used in calculating diluted earnings per share	244,966,000	261,658,000	244,899,000	267,272,000
Numerator:
Net income allocable to preferred stock	$ 0	$ 600,000	$ 500,000	$ 900,000
Denominator:
Total	0	7,200,000	200,000	6,600,000
Stock options and employee stock purchase plan [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	1,000,000	800,000	1,300,000	900,000
Denominator:
Total	0	5,300,000		5,100,000
Market stock units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	200,000		200,000
Denominator:
Total	0	0	0	0
Time Vested Restricted Stock Units [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	1,400,000	1,000,000	1,500,000	1,400,000
Denominator:
Total		1,400,000		1,000,000
Performance vested restricted stock units settled in shares [Member]
Effect of dilutive securities:
Stock options and employee stock purchase plan	0		0
Denominator:
Total	0	0	0	0
Convertible preferred stock [Member]
Denominator:
Total	0	500,000	200,000	500,000

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Share-Based Payments (Details)	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010
Schedule of stock option and restricted stock unit grants
Stock options		124,000
Market stock units [Member]
Schedule of stock option and restricted stock unit grants
Restricted stock units	373,000	334,000
Cash settled performance shares [Member]
Schedule of stock option and restricted stock unit grants
Restricted stock units	483,000	373,000
Time Vested Restricted Stock Units [Member]
Schedule of stock option and restricted stock unit grants
Restricted stock units	1,303,000	1,700,000
Performance vested restricted stock units settled in shares [Member]
Schedule of stock option and restricted stock unit grants
Restricted stock units	1,000	4,000

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Share-Based Payments (Details 1) (USD  $) In Millions, except Share data	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses	$ 36.1	$ 49.4	$ 74.4	$ 102.3
Capitalized share-based payment costs	(1)	(0.7)	(2)	(1.6)
Share-based compensation expense included in total costs and expenses	35.1	48.7	72.4	100.7
Income tax effect	(10.9)	(15.7)	(23)	(32.4)
Share-Based Payments (Textuals) [Abstract]
Shares issued under ESPP			316,000	335,000
Time Vested Restricted Stock Units [Member] | February 2011 [Member]
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			1.2
Market stock units [Member] | Attainment of performance criteria [Member]
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			18,000
Cash settled performance shares [Member] | Attainment of performance criteria [Member]
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			95,000
Time Vested Restricted Stock Units [Member] | Attainment of performance criteria [Member]
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			135,000
Attainment of performance criteria [Member] | Performance vested restricted stock units [Member]
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			1,000
Research and development [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses	13.9	15.4	32.2	32.1
Selling, general and administrative [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses	22.2	34	42.8	70.2
Restructuring Charges [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses			(0.6)
Total share-based compensation expense, net of tax [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses	24.2	33	49.4	68.3
Time Vested Restricted Stock Units [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses	19.1	32.3	46.2	65.8
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			1,303,000	1,700,000
Market stock units [Member]
Share-based Compensation Expense included in consolidated statements of income
Share-based compensation expense included in total costs and expenses	$ 4.3	$ 1.9	$ 7.7	$ 5.5
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			373,000	334,000
Cash settled performance shares [Member]
Additional-Share Based Payments (Textuals) [Abstract]
Stock granted in connection with annual awards made in February 2011			483,000	373,000

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Share-Based Payments (Details 2) (USD  $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	$ 36.1	$ 49.4	$ 74.4	$ 102.3
Capitalized share-based compensation costs	(1)	(0.7)	(2)	(1.6)
Share-based compensation expense included in total costs and expenses	35.1	48.7	72.4	100.7
Stock Options [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	1.6	9.3	2.7	20.1
Market stock units [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	4.3	1.9	7.7	5.5
Time Vested Restricted Stock Units [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	19.1	32.3	46.2	65.8
Performance vested restricted stock units settled in shares [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	0.3	1.3	0.7	3.7
Cash settled performance shares [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	10.7	4.2	15.5	5.2
Employee Stock Purchase Plan [Member]
Summary of share based compensation expense associated with different programs
Share-based compensation expense included in total costs and expenses	$ 0.1	$ 0.4	$ 1.6	$ 2

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Income Taxes (Details)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Reconciliation between the U.S. federal statutory tax rate and effective tax rate
Statutory rate	35.00%	35.00%	35.00%	35.00%
State taxes	0.40%	1.90%	1.40%	1.90%
Taxes on foreign earnings	(7.30%)	(10.30%)	(6.30%)	(10.10%)
Credits and net operating loss utilization	(4.10%)	(1.60%)	(3.10%)	(1.60%)
Purchased intangible assets	1.40%	1.50%	1.40%	1.50%
IPR&D		0.80%		0.80%
Permanent items	(1.10%)	(1.70%)	(1.20%)	(1.70%)
Other	(0.50%)	0.20%	(1.50%)	(0.10%)
Effective tax rate	23.80%	25.80%	25.70%	25.70%

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Income Taxes (Details) (Textual) (USD  $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Jun. 08, 2010	Dec. 31, 2006
Income Taxes (Textuals) [Abstract]
Effective tax rate	23.80%	25.80%	25.70%	25.70%
Notice of Assessment of corporate excise tax including penalties and interest	$ 142.4		$ 142.4		$ 103.5	$ 38.9
Amount for settlement of exchange for payment of tax			$ 7

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Other Consolidated Financial Statement Detail (Details) (USD  $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Other income (expense), net
Interest income	$ 4,300,000	$ 6,700,000	$ 8,000,000	$ 15,600,000
Interest expense	(8,400,000)	(9,000,000)	(17,600,000)	(17,300,000)
Impairments of investments	(5,500,000)	(1,200,000)	(6,800,000)	(17,000,000)
Foreign exchange gain (losses), net	(600,000)	(700,000)	(1,000,000)	300,000
Gain (loss) on sales of investments, net	200,000	6,300,000	15,500,000	11,300,000
Other, net	(1,700,000)	(1,100,000)	100,000	(300,000)
Total other income (expense), net	(11,728,000)	1,012,000	(1,777,000)	(7,373,000)
Other current assets
Deferred tax assets	66,900,000		66,900,000		112,200,000
Prepaid taxes	33,100,000		33,100,000		31,400,000
Receivable from collaborations	8,800,000		8,800,000		7,300,000
Interest receivable	6,100,000		6,100,000		4,900,000
Other prepaid expenses	60,100,000		60,100,000		47,900,000
Other	26,800,000		26,800,000		12,100,000
Total other current assets	201,794,000		201,794,000		215,822,000
Accrued Expenses and Other
Employee compensation and benefits	134,300,000		134,300,000		159,700,000
Revenue-related rebates	112,400,000		112,400,000		105,300,000
Restructuring charges	8,900,000		8,900,000		66,400,000
Royalties and licensing fees	40,000,000		40,000,000		45,100,000
Deferred revenue	59,100,000		59,100,000		41,300,000
Collaboration expenses	57,100,000		57,100,000		31,600,000
Clinical development expenses	33,700,000		33,700,000		24,400,000
Interest payable	21,600,000		21,600,000		21,600,000
Construction in progress accrual	13,300,000		13,300,000		16,400,000
Current portion of contingent consideration	4,900,000		4,900,000		11,900,000
Derivative liability	24,900,000		24,900,000		12,200,000
Other	131,100,000		131,100,000		130,000,000
Total accrued expenses and other	$ 641,273,000		$ 641,273,000		$ 665,923,000

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Investments in Variable Interest Entities (Details) (USD  $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Joint venture assets and liabilities
Assets	$ 8,621,979	$ 8,092,493
Liabilities	2,565,785	2,643,050
Corporate Joint Venture [Member]
Joint venture assets and liabilities
Assets	193,600	159,200
Liabilities	$ 74,600	$ 63,300

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Investments in Variable Interest Entities (Details 1) (USD  $)	6 Months Ended						3 Months Ended	6 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended
Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Corporate Joint Venture [Member]	Dec. 31, 2010 Corporate Joint Venture [Member]	Jun. 30, 2011 Corporate Joint Venture [Member] Biogen Dompe SRL [Member]	Dec. 31, 2010 Corporate Joint Venture [Member] Biogen Dompe SRL [Member]	Jun. 30, 2011 Neurimmune [Member]	Jun. 30, 2010 Neurimmune [Member]	Jun. 30, 2011 Neurimmune [Member]	Jun. 30, 2010 Neurimmune [Member]	Aug. 31, 2010 Knopp [Member]	Jun. 30, 2011 Knopp [Member]	Mar. 31, 2011 Knopp [Member]	Jun. 30, 2011 Knopp [Member]
Investment in Variable Interest Entities (Textuals) [Abstract]
Account receivable from ordinary course of business related to Joint Ventures	$ 666,960,000	$ 605,329,000	$ 134,200,000	$ 124,200,000	$ 127,800,000	$ 118,000,000
Total upfront and reimbursement payment made to collaborative partner											26,400,000
Remaining potential development milestone payments and royalties on commercial sales under the terms of collaboration agreement									345,000,000		265,000,000
Purchase of Class B Common Shares in Variable Interest Entities											30.00%
Purchase Consideration for Variable Interest Entities											60,000,000
Payment recognized as a charge to noncontrolling interests, net of tax									15,000,000				10,000,000
Biogen Idec's share of research and development expense reflected within our research and development expense							3,000,000	5,300,000	4,800,000	10,400,000		9,000,000		14,700,000
Percentage of funding for R&D cost required in support of the collaboration agreement									100.00%
Percentage of consolidated operations of sales affiliates	100.00%
Investment in biotechnology companies that are determined to be unconsolidated variable interest entities	$ 17,700,000	$ 22,900,000

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Litigation (Details) (USD  $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2011	Jun. 08, 2010	Dec. 31, 2006
Litigation Details (Textuals) [Abstract]
Notice of Assessment under consideration for corporate excise tax	$ 142.4	$ 103.5	$ 38.9
Stipulated royalty rate	0.50%
Decrease in share of co-promotion profits due to estimated compensation damages	$ 50
Number of consolidated lawsuits	2
Maximum percentage estimation range of potential loss for royalty as percentage of net sales	0.50%

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Segment Information (Details)	6 Months Ended
Jun. 30, 2011
Segment Information (Textuals) [Abstract]
Number of reportable segment	1

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