Trading Cash Instruments - Cash Instruments by Level (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Trading Cash Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Trading Assets	$ 343,880	$ 310,080
Trading cash instruments assets | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	241,412	222,413
Trading cash instruments assets | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	101,203	86,425
Trading cash instruments assets | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	1,265	1,242
Trading cash instruments assets | U.S. Government and Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	136,545	142,935
Trading cash instruments assets | U.S. Government and Agency Obligations [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	97,256	108,200
Trading cash instruments assets | U.S. Government and Agency Obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	39,289	34,714
Trading cash instruments assets | U.S. Government and Agency Obligations [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value		21
Trading cash instruments assets | Non-U.S. Government and Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	70,438	44,839
Trading cash instruments assets | Non-U.S. Government and Agency Obligations [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	60,431	33,709
Trading cash instruments assets | Non-U.S. Government and Agency Obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	9,991	11,108
Trading cash instruments assets | Non-U.S. Government and Agency Obligations [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	16	22
Trading cash instruments assets | Loans and Securities Backed by Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	1,093	2,222
Trading cash instruments assets | Loans and Securities Backed by Commercial Real Estate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	897	2,031
Trading cash instruments assets | Loans and Securities Backed by Commercial Real Estate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	196	191
Trading cash instruments assets | Loans and Securities Backed by Residential Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	5,683	6,025
Trading cash instruments assets | Loans and Securities Backed by Residential Real Estate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	5,503	5,794
Trading cash instruments assets | Loans and Securities Backed by Residential Real Estate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	180	231
Trading cash instruments assets | Corporate debt instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	33,257	28,773
Trading cash instruments assets | Corporate debt instruments [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	809	1,313
Trading cash instruments assets | Corporate debt instruments [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	31,666	26,768
Trading cash instruments assets | Corporate debt instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	782	692
Trading cash instruments assets | State and municipal obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	325	680
Trading cash instruments assets | State and municipal obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	325	680
Trading cash instruments assets | Other debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	2,623	1,493
Trading cash instruments assets | Other debt obligations [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	278	409
Trading cash instruments assets | Other debt obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	2,318	1,074
Trading cash instruments assets | Other debt obligations [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	27	10
Trading cash instruments assets | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	85,024	79,346
Trading cash instruments assets | Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	82,638	78,782
Trading cash instruments assets | Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	2,322	489
Trading cash instruments assets | Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	64	75
Trading cash instruments assets | Commodities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	8,892	3,767
Trading cash instruments assets | Commodities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Assets at Fair Value	8,892	3,767
Cash Instruments Liabilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(105,443)	(65,033)
Cash Instruments Liabilities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(94,581)	(54,761)
Cash Instruments Liabilities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(10,594)	(9,999)
Cash Instruments Liabilities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(268)	(273)
Cash Instruments Liabilities [Member] | U.S. Government and Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(22,089)	(9,961)
Cash Instruments Liabilities [Member] | U.S. Government and Agency Obligations [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(22,084)	(9,914)
Cash Instruments Liabilities [Member] | U.S. Government and Agency Obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(5)	(47)
Cash Instruments Liabilities [Member] | Non-U.S. Government and Agency Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(29,273)	(23,424)
Cash Instruments Liabilities [Member] | Non-U.S. Government and Agency Obligations [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(27,700)	(21,213)
Cash Instruments Liabilities [Member] | Non-U.S. Government and Agency Obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(1,572)	(2,205)
Cash Instruments Liabilities [Member] | Non-U.S. Government and Agency Obligations [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(1)	(6)
Cash Instruments Liabilities [Member] | Loans and Securities Backed by Commercial Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(7)	(32)
Cash Instruments Liabilities [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(7)	(31)
Cash Instruments Liabilities [Member] | Loans and Securities Backed by Commercial Real Estate [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value		(1)
Cash Instruments Liabilities [Member] | Loans and Securities Backed by Residential Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(1)	(2)
Cash Instruments Liabilities [Member] | Loans and Securities Backed by Residential Real Estate [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(1)	(2)
Cash Instruments Liabilities [Member] | Corporate debt instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(8,438)	(7,862)
Cash Instruments Liabilities [Member] | Corporate debt instruments [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(25)	(115)
Cash Instruments Liabilities [Member] | Corporate debt instruments [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(8,173)	(7,494)
Cash Instruments Liabilities [Member] | Corporate debt instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(240)	(253)
Cash Instruments Liabilities [Member] | State and municipal obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value		(2)
Cash Instruments Liabilities [Member] | State and municipal obligations [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value		(2)
Cash Instruments Liabilities [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(45,635)	(23,744)
Cash Instruments Liabilities [Member] | Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(44,772)	(23,519)
Cash Instruments Liabilities [Member] | Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	(836)	(212)
Cash Instruments Liabilities [Member] | Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value	$ (27)	(13)
Cash Instruments Liabilities [Member] | Commodities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value		(6)
Cash Instruments Liabilities [Member] | Commodities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Cash Instruments Liabilities at Fair Value		$ (6)