Collateralized Agreements and Financings - Financial Instruments Received as Collateral and Repledged (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Collateralized Agreements And Financings [Abstract]
Securities segregated for regulatory and other purposes	$ 40,300	$ 26,760

Fair value of securities deposited in a special reserve account for the exclusive benefit of customers.

No definition available.

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