Deferred Compensation Arrangement Vesting Period

No definition available.

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

- Definition

Deferred compensation plan, annual percentage of incentive compensation deferred, maximum

+ References

No definition available.

+ Details

Name:	blk_DeferredCompensationPlanAnnualPercentageOfIncentiveCompensationDeferredMaximum
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Deferred compensation plan, annual percentage of incentive compensation deferred, minimum

+ References

No definition available.

+ Details

Name:	blk_DeferredCompensationPlanAnnualPercentageOfIncentiveCompensationDeferredMinimum
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Deferred compensation plan, deferral period, years

+ References

No definition available.

+ Details

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

- Definition

Defined Contribution Plan Employee Contribution Percentage of Employee Compensation Maximum

+ References

No definition available.

+ Details

Name:	blk_DefinedContributionPlanEmployeeContributionPercentageOfEmployeeCompensationMaximum
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan, employer matching annual contribution, percentage of eligible compensation, maximum

+ References

No definition available.

+ Details

Name:	blk_DefinedContributionPlanEmployerMatchingAnnualContributionPercentageOfEligibleCompensationMaximum
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan, employer matching annual contribution, percentage of eligible compensation, minimum

+ References

No definition available.

+ Details

Name:	blk_DefinedContributionPlanEmployerMatchingAnnualContributionPercentageOfEligibleCompensationMinimum
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan employer matching percentage of employee contribution.

+ References

No definition available.

+ Details

Name:	blk_DefinedContributionPlanEmployerMatchingPercentageOfEmployeeContribution
Namespace Prefix:	blk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined contribution plan maximum annual contribution amounts by employer.

+ References

No definition available.

+ Details

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

- Definition

Employee benefit plans.

+ References

No definition available.

+ Details

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

- Definition

Number Of Annual Installments

+ References

No definition available.

+ Details

Name:	blk_NumberOfAnnualInstallments
Namespace Prefix:	blk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements. Represents currently earned compensation under compensation arrangements that is not actually paid until a later date.

+ References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) during the reporting period in the obligation created by employee agreements whereby earned compensation will be paid in the future.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInDeferredCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for deferred compensation arrangements other than those specified elsewhere in the taxonomy.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_PlanNameAxis=blk_DeferredCompensationPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=blk_VoluntaryDeferredCompensationPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=blk_RabbiTrustMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=blk_DefinedContributionPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=us-gaap_ForeignPensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=blk_OtherRegionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=blk_DefinedBenefitPlansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=blk_JapanAndGermanyPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Employee Benefit Plans - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Jan. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) installment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Deferred Compensation Plans [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Other deferred compensation plan liability		$ 178,000,000	$ 126,000,000
Deferred Compensation Plans [Member] \| Subsequent Event [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Additional deferred compensation granted	$ 151,000,000
Deferred compensation, vesting period	3 years
Deferred Compensation Plans [Member] \| Voluntary Deferred Compensation Plan [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Deferred compensation plan, annual percentage of incentive compensation deferred, minimum		1.00%
Deferred compensation plan, annual percentage of incentive compensation deferred, maximum		100.00%
Deferred compensation plan, deferral period, years		up to 10 years
Deferred compensation plan, number of annual installments \| installment		10
Deferred Compensation Plans [Member] \| Rabbi Trust [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Deferred compensation plan assets		$ 65,000,000	65,000,000
Deferred compensation plan liability		$ 88,000,000	78,000,000
Defined Contribution Plans [Member] \| U.K. Plan [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Defined contribution plan, employee contribution, percentage of employee compensation, maximum		15.00%
Defined contribution plan, employer matching annual contribution, percentage of eligible compensation, minimum		6.00%
Defined contribution plan expense		$ 33,000,000	33,000,000	$ 29,000,000
Defined Contribution Plans [Member] \| Other Regions [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Defined contribution plan expense		$ 18,000,000	18,000,000	18,000,000
Defined Contribution Plans [Member] \| U.S. Plan [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Defined contribution plan, employee contribution, percentage of employee compensation, maximum		8.00%
Defined Contribution plan, employer matching contribution percentage of employee contribution		50.00%
Defined contribution plan, employer matching annual contribution, percentage of eligible compensation, minimum		3.00%
Defined contribution plan, employer matching annual contribution, percentage of eligible compensation, maximum		5.00%
Defined contribution plan, employer matching contribution, maximum amount		$ 5,000
Defined contribution plan expense		72,000,000	67,000,000	$ 63,000,000
Defined Benefit Plans [Member] \| Japan and Germany Plan [Member]
Deferred Compensation Plans, Defined Contribution Plans and Defined Benefit Plans [Line Items]
Deferred compensation plan assets		$ 22,000,000	$ 21,000,000