Income Taxes(Narrative) (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended
Apr. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income Taxes [Line Items]
Net non-cash tax benefit		$ 30
Non-cash income tax benefit related to revaluation of deferred income tax assets and liabilities	$ 52		$ 129

Tax benefit due to revaluation of net deferred income tax liabilities.

No definition available.

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Namespace Prefix:	blk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration