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BIOGEN INC.

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 2.2.0.7 false Other Income (Expense), Net (Details) (USD $) 0615 - Disclosure - Other Income (Expense), Net (Details) true false false false 1 USD false false USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 Pure Standard http://www.xbrl.org/2003/instance pure xbrli 0 USDEPS Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 $ false 2 USD false false Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 Pure Standard http://www.xbrl.org/2003/instance pure xbrli 0 USDEPS Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 $ false 3 USD false false USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 Pure Standard http://www.xbrl.org/2003/instance pure xbrli 0 USDEPS Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 $ false 4 USD false false Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 Pure Standard http://www.xbrl.org/2003/instance pure xbrli 0 USDEPS Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 $ 3 1 biib_OtherIncomeExpenseNetAbstract biib false na duration Other Income (Expense), Net. false false false false false true false false false false false verboselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false 4 false false false false 0 0 false false false xbrli:stringItemType string Other Income (Expense), Net. false 4 2 us-gaap_InvestmentIncomeInterest us-gaap true credit duration No definition available. false false false false false false false false false false false verboselabel false 1 true true false false 3100000 3100000 false false false 2 true true false false 10900000 10900000 false false false 3 true true false false 18600000 18600000 false false false 4 true true false false 37800000 37800000 false false false xbrli:monetaryItemType monetary Income derived from investments in debt securities and on cash and cash equivalents the earnings of which reflect the time value of money or transactions in which the payments are for the use or forbearance of money. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 14 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 7 -Article 5 false 5 2 us-gaap_InterestExpense us-gaap true debit duration No definition available. false false false false false false false false false false true negated false 1 false true false false -9300000 -9300000 false false false 2 false true false false -8500000 -8500000 false false false 3 false true false false -26600000 -26600000 false false false 4 false true false false -27600000 -27600000 false false false xbrli:monetaryItemType monetary The cost of borrowed funds accounted for as interest that was charged against earnings during the period. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 34 -Paragraph 21 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher OTS -Name Federal Regulation (FR) -Number Title 12 -Chapter V -Section 563c.102 -Paragraph 9 -Subsection II Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 9 -Article 9 false 6 2 us-gaap_ImpairmentOfInvestments us-gaap true debit duration No definition available. false false false false false false false false false false true negated false 1 false true false false -2800000 -2800000 false false false 2 false true false false -500000 -500000 false false false 3 false true false false -19800000 -19800000 false false false 4 false true false false -10100000 -10100000 false false false xbrli:monetaryItemType monetary This element represents the amount by which the carrying amount exceeds the fair value of the investment. The amount is charged to income if the decline in fair value is deemed to be other than temporary. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 16 false 7 2 us-gaap_ForeignCurrencyTransactionGainLossBeforeTax us-gaap true credit duration No definition available. false false false false false false false false false false false verboselabel false 1 false true false false -3500000 -3500000 false false false 2 false true false false 3200000 3200000 false false false 3 false true false false -3200000 -3200000 false false false 4 false true false false 10600000 10600000 false false false xbrli:monetaryItemType monetary The aggregate foreign currency transaction gain or loss (both realized and unrealized) included in determining net income for the reporting period. Excludes foreign currency transactions designated as hedges of net investment in a foreign entity and intercompany foreign currency transactions that are of a long-term nature, when the entities to the transaction are consolidated, combined, or accounted for by the equity method in the reporting enterprise's financial statements. For certain enterprises, primarily banks, that are dealers in foreign exchange, foreign currency transaction gains or losses may be disclosed as dealer gains or losses. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 52 -Paragraph 30 false 8 2 biib_NetRealizedGainLossOnMarketableSecurities biib false credit duration This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable... false false false false false false false false false false false verboselabel false 1 false true false false 4800000 4800000 false false false 2 false true false false 1800000 1800000 false false false 3 false true false false 16100000 16100000 false false false 4 false true false false 13700000 13700000 false false false xbrli:monetaryItemType monetary This item represents the total realized gain (loss) included in earnings for the period as a result of selling marketable securities categorized as trading, available-for-sale, or held-to-maturity. No authoritative reference available. false 9 2 biib_OtherIncomeExpenseNet biib false credit duration The amount of other nonoperating income and expense, which does not qualify for separate disclosure on the income statement. false false false false false false false false false false false totallabel false 1 false true false false 800000 800000 false false false 2 false true false false 2500000 2500000 false false false 3 false true false false 600000 600000 false false false 4 false true false false 6500000 6500000 false false false xbrli:monetaryItemType monetary The amount of other nonoperating income and expense, which does not qualify for separate disclosure on the income statement. No authoritative reference available. true 10 2 us-gaap_OtherNonoperatingIncomeExpense us-gaap true credit duration No definition available. false false false false false false false false false false false verboselabel false 1 true true false false -6945000 -6945000 false false false 2 true true false false 9360000 9360000 false false false 3 true true false false -14318000 -14318000 false false false 4 true true false false 30886000 30886000 false false false xbrli:monetaryItemType monetary The net amount of other nonoperating income and expense, which does not qualify for separate disclosure on the income statement under materiality guidelines. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 9 -Article 5 false 4 8 false NoRounding UnKnown UnKnown false true