Credit Concentrations - Credit Concentration, Resale Agreements and Securities Borrowed (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
U.S. Government and Agency Obligations that Collateralize Securities Purchased Under Agreements to Resell and Securities Borrowed [Member]
Concentration Risk [Line Items]
Concentration risk, Credit risk, Financial instrument, Maximum exposure	$ 56,896	$ 49,396
Non-U.S. Government and Agency Obligations that Collateralize Securities Purchased Under Agreements to Resell and Securities Borrowed [Member]
Concentration Risk [Line Items]
Concentration risk, Credit risk, Financial instrument, Maximum exposure	$ 60,885	$ 55,889

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

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