Fair value of restricted stock units and restricted stock granted to employees.

No definition available.

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

- Definition

Remaining share of committed long-term incentive awards.

+ References

No definition available.

+ Details

Name:	blk_RemainingShareOfCommittedLongTermIncentiveAwards
Namespace Prefix:	blk_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Share based compensation arrangement by share based payment award equity instruments other than options granted in period fair value.

+ References

No definition available.

+ Details

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

- Definition

Share based compensation arrangement by share based payment award equity instruments other than options surrendered in period

+ References

No definition available.

+ Details

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

- Definition

Share based compensation arrangement by share based payment award equity instruments other than options surrendered number.

+ References

No definition available.

+ Details

Name:	blk_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsSurrenderedNumber
Namespace Prefix:	blk_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Share based compensation arrangement by share based payment award equity instruments other than options that cliff vest.

+ References

No definition available.

+ Details

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

- Definition

Shares committed to fund long term incentive plans

+ References

No definition available.

+ Details

Name:	blk_SharesCommittedToFundLongTermIncentivePlans
Namespace Prefix:	blk_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

+ References

+ Details

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

- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the equity-based award expires, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpirationDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of vesting of share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Intrinsic value of equity-based compensation awards outstanding. Excludes stock and unit options.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_AwardTypeAxis=blk_RestrictedStockUnitsAndRestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=blk_PerformanceBasedRestrictedStockUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=blk_MarketPerformanceBasedRestrictedStockUnitsMember
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_AwardTypeAxis=blk_LongTermIncentivePlansFundedByPncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended
	Jan. 31, 2017	Mar. 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock and RSUs, Granted			1,032,221
Fair value of RSUs/restricted stock granted to employees			$ 388
RSUs/Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock and RSUs, Granted	699,991
Restricted stock units vesting period, years	3 years
Awards to employees cliff vesting	277,313
RSUs to employees that cliff vest, percentage	100.00%
RSUs to employees that cliff vest, date	Jan. 31, 2020
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of outstanding RSUs			$ 1,100
Stock price			$ 422.41
Unrecognized stock-based compensation expense			$ 434
Remaining weighted-average period			1 year 4 months 24 days
Performance-Based RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted Stock and RSUs, Granted	293,385		294,584
Restricted stock units vesting period, years	3 years
Awards to employees cliff vesting	293,385
RSUs to employees that cliff vest, percentage	100.00%
RSUs to employees that cliff vest, date	Jan. 31, 2020
Intrinsic value of outstanding RSUs			$ 382
Stock price			$ 422.41
Unrecognized stock-based compensation expense			$ 157
Remaining weighted-average period			1 year 8 months 12 days
Fair value of RSUs/restricted stock granted to employees			$ 111
Market Performance-Based RSUs [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units vesting period, years			6 years	6 years	6 years
Intrinsic value of outstanding RSUs			$ 121
Stock price			$ 422.41
Unrecognized stock-based compensation expense			$ 7
Market Performance-Based RSUs [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Remaining weighted-average period			1 year
Long-Term Incentive Plans Funded by PNC [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares surrendered			3,800,000
Number of additional shares surrendered		517,138
Remaining shares committed by PNC for long-term incentive awards			200,000
Long-Term Incentive Plans Funded by PNC [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares committed to fund long-term incentive plans			4,000,000