Equity method investments shares owned.

No definition available.

Name:	blk_EquityMethodInvestmentsSharesOwned
Namespace Prefix:	blk_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Operating expenses, offset amount by noncash non operating pre-tax gain on sale of equity method investments.

+ References

No definition available.

+ Details

Name:	blk_OperatingExpensesOffsetAmountByNoncashNonOperatingPreTaxGainOnSaleOfEquityMethodInvestments
Namespace Prefix:	blk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Operating expenses related to charitable contribution.

+ References

No definition available.

+ Details

Name:	blk_OperatingExpensesRelatedToCharitableContribution
Namespace Prefix:	blk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Other Assets [Line Items]

+ References

No definition available.

+ Details

Name:	blk_OtherAssetsLineItems
Namespace Prefix:	blk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Tax effects of discrete items recognized in connection with charitable contribution.

+ References

No definition available.

+ Details

Name:	blk_TaxEffectsOfDiscreteItemsRecognizedInConnectionWithCharitableContribution
Namespace Prefix:	blk_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109237563&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 25
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8813-108599

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_EquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of investments accounted under the equity method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=117311354&loc=d3e32014-111567

+ Details

Name:	us-gaap_EquityMethodInvestmentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of income (expense) related to nonoperating activities, classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

Name:	us-gaap_OtherNonoperatingIncomeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=blk_PennyMacFinancialServicesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=blk_BlackRockFoundationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=blk_IcapitalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Other Assets - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Feb. 13, 2020
Other Assets [Line Items]
Tax benefit	$ (241)
PennyMac [Member]
Other Assets [Line Items]
Ownership percentage		20.00%	20.00%
Shares		16
Carrying value - equity method investment		$ 451
Market value of equity method investments		$ 530
PennyMac [Member] \| BlackRock Foundation [Member]
Other Assets [Line Items]
Operating expenses related to charitable contribution	589
Operating expenses, offset amount by noncash non operating pre-tax gain on sale of equity method investments.	122
Tax benefit	241
iCapital [Member]
Other Assets [Line Items]
Carrying value - equity method investment	296
Nonoperating pre-tax gain	$ 240