Fair Value Disclosures - Investments in Certain Entities Calculate Net Asset Value per Share (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2017	Dec. 31, 2016
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 760	$ 551
Total Unfunded Commitments	287	157
Hedge Funds/Funds of Hedge Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 22	$ 36
Hedge Funds/Funds of Hedge Funds [Member] \| Maximum [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	90 days	90 days
Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Total Unfunded Commitments	$ 167	$ 56
Real Assets Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	42	27
Total Unfunded Commitments	54	21
Private Equity Funds Of Funds [Member] \| Consolidated Variable Interest Entities [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	77	89
Total Unfunded Commitments	20	16
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	253	237
Total Unfunded Commitments	$ 11	$ 14
Redemption Frequency (Daily)	21.00%	21.00%
Redemption Frequency (Monthly)	21.00%	21.00%
Redemption Frequency (Quarterly)	54.00%	51.00%
Redemption Frequency (Not Redeemable)	25.00%	28.00%
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member] \| Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	1 day	1 day
Equity Method Investments [Member] \| Hedge Funds/Funds of Hedge Funds [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	90 days	90 days
Equity Method Investments [Member] \| Private Equity Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 87	$ 90
Total Unfunded Commitments	67	62
Equity Method Investments [Member] \| Real Assets Funds [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	261	60
Total Unfunded Commitments	$ 113	$ 35
Redemption Frequency (Quarterly)	86.00%	41.00%
Redemption Frequency (Not Redeemable)	14.00%	59.00%
Redemption Notice Period, Not Redeemable	60 days	60 days
Equity Method Investments [Member] \| Other Types Of Investments [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 18	$ 12
Total Unfunded Commitments	$ 22	$ 9
Redemption Frequency (Daily)	62.00%	42.00%
Redemption Frequency (Monthly)	62.00%	42.00%
Redemption Frequency (Not Redeemable)	38.00%	58.00%
Equity Method Investments [Member] \| Other Types Of Investments [Member] \| Minimum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	3 days	3 days
Equity Method Investments [Member] \| Other Types Of Investments [Member] \| Maximum [Member]
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Redemption Notice Period, days	5 days	5 days

Fair Value Measured Recurring Basis Using Net Asset Per Share Value Redemption Frequency (daily)

No definition available.

Name:	blk_FairValueMeasuredRecurringBasisUsingNetAssetPerShareValueRedemptionFrequencyDaily
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair Value Measured Recurring Basis Using Net Asset Per Share Value Redemption Frequency (monthly)

+ References

No definition available.

+ Details

Name:	blk_FairValueMeasuredRecurringBasisUsingNetAssetPerShareValueRedemptionFrequencyMonthly
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value measured recurring basis using net asset per share value redemption frequency not redeemable.

+ References

No definition available.

+ Details

Name:	blk_FairValueMeasuredRecurringBasisUsingNetAssetPerShareValueRedemptionFrequencyNotRedeemable
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair Value Measured Recurring Basis Using Net Asset Per Share Value Redemption Frequency (quarterly)

+ References

No definition available.

+ Details

Name:	blk_FairValueMeasuredRecurringBasisUsingNetAssetPerShareValueRedemptionFrequencyQuarterly
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair Value Measured Recurring Basis Using Net Asset Per Share Value Redemption Notice Period Not Redeemable

+ References

No definition available.

+ Details

Name:	blk_FairValueMeasuredRecurringBasisUsingNetAssetPerShareValueRedemptionNoticePeriodNotRedeemable
Namespace Prefix:	blk_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

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

- Definition

Notice period the entity is required to deliver before it can redeem an investment calculated using net asset value per share, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Includes, but is not limited to, investments calculated by per unit, per membership interest, other equity or ownership interest and alternative investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareInvestmentRedemptionNoticePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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

- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=SL6742756-110258

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_HedgeFundsEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryAggregatedDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_RealEstateFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=blk_PrivateEquityFundsOfFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_EquityMethodInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=blk_OtherTypesOfInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: