Fair Value Option - Financial Assets and Financial Liabilities by Level (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Resale agreements	$ 101,277		$ 85,691
Securities borrowed	31,680		26,279
Customer and other receivables	60		53
Total financial assets at fair value	133,017		112,023
Deposits	(18,401)		(17,765)
Repurchase agreements	(101,279)		(117,756)
Securities loaned	(1,060)		(714)
Other secured financings	(23,565)		(18,071)
Unsecured borrowings Short-term	(25,930)		(26,007)
Unsecured borrowings Long-term	(40,328)		(43,661)
Other liabilities	(336)		(150)
Total financial liabilities at fair value	(210,899)		(224,124)
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Resale agreements	101,277		85,691
Securities borrowed	31,680		26,279
Customer and other receivables	60		53
Total financial assets at fair value	133,017		112,023
Deposits	(14,251)		(13,742)
Repurchase agreements	(101,277)		(117,726)
Securities loaned	(1,060)		(714)
Other secured financings	(20,502)		(17,685)
Unsecured borrowings Short-term	(18,356)		(20,300)
Unsecured borrowings Long-term	(27,548)		(32,920)
Other liabilities	(1)		(1)
Total financial liabilities at fair value	(182,995)		(203,088)
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial assets at fair value	27,677	$ 29,426	23,068
Deposits	(4,150)		(4,023)
Repurchase agreements	(2)		(30)
Other secured financings	(3,063)		(386)
Unsecured borrowings Short-term	(7,574)		(5,707)
Unsecured borrowings Long-term	(12,780)		(10,741)
Other liabilities	(335)		(149)
Total financial liabilities at fair value	$ (27,904)		$ (21,036)