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Leases - Schedule of Supplemental Information Related to Operating Leases (Parenthetical) (Detail) - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Operating Leased Assets [Line Items] |  |
Operating lease ROU assets | $ 1,874 | $ 1,519
Global Infrastructure Management, LLC [Member] |  |
Operating Leased Assets [Line Items] |  |
Operating lease ROU assets | $ 178 | $ 75

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 842 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479041/842-20-45-1 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=blk_GlobalInfrastructureManagementLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |