Debt instrument unamortized discount and debt issuance costs.

No definition available.

Name:	blk_DebtInstrumentUnamortizedDiscountAndDebtIssuanceCosts
Namespace Prefix:	blk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=blk_FivePercentNotesDueTwoThousandNineteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=blk_FourPointTwoFivePercentNotesDueTwoThousandTwentyOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=blk_ThreePointThreeSevenFivePercentNotesDueTwoThousandTwentyTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=blk_ThreePointFiveZeroPercentNotesDueTwoThousandTwentyFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=blk_OnePointTwoFivePercentNotesDueTwoThousandTwentyFiveMember
Namespace Prefix:
Data Type:	na
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Borrowings - Carrying Value and Fair Value of Long-Term Borrowings (Detail) € in Millions, $ in Millions	Dec. 31, 2015 USD ($)	May. 06, 2015 EUR (€)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Maturity Amount	$ 4,960
Unamortized Discount and Debt Issuance Costs	(30)
Carrying Value	4,930		$ 4,922
Fair Value	5,223		$ 5,309
6.25% Notes due 2017 [Member]
Debt Instrument [Line Items]
Maturity Amount	700
Unamortized Discount and Debt Issuance Costs	(1)
Carrying Value	699
Fair Value	757
5.00% Notes due 2019 [Member]
Debt Instrument [Line Items]
Maturity Amount	1,000
Unamortized Discount and Debt Issuance Costs	(3)
Carrying Value	997
Fair Value	1,106
4.25% Notes due 2021 [Member]
Debt Instrument [Line Items]
Maturity Amount	750
Unamortized Discount and Debt Issuance Costs	(5)
Carrying Value	745
Fair Value	828
3.375% Notes due 2022 [Member]
Debt Instrument [Line Items]
Maturity Amount	750
Unamortized Discount and Debt Issuance Costs	(6)
Carrying Value	744
Fair Value	773
3.50% Notes due 2024 [Member]
Debt Instrument [Line Items]
Maturity Amount	1,000
Unamortized Discount and Debt Issuance Costs	(8)
Carrying Value	992
Fair Value	1,030
1.25% Notes due 2025 [Member]
Debt Instrument [Line Items]
Maturity Amount	760	€ 700
Unamortized Discount and Debt Issuance Costs	(7)
Carrying Value	753
Fair Value	$ 729