Fair Value Option - Components of Gain (Loss) on Instruments at Fair Value and Related Derivatives (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Mark-to-market on related derivatives		$ 193	$ 350	$ (209)	$ 917
Gain (loss) on instruments designated at fair value and related derivatives		29	7	63	40
Gain (Loss) on Instruments Designated at Fair Value and Related Derivatives [Member]
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Interest rate and other components	[1]	(178)	(356)	251	(920)
Credit risk component, total	[2]	0	(1)	3	(3)
Total mark-to-market on financial instruments designated at fair value		(178)	(357)	254	(923)
Mark-to-market on related derivatives		197	348	(225)	918
Net realized gain on related long-term debt derivatives		10	16	34	45
Gain (loss) on instruments designated at fair value and related derivatives		29	7	63	40
Gain (Loss) on Instruments Designated at Fair Value and Related Derivatives [Member] | Loans [Member]
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Interest rate and other components	[1]	0	0	0	0
Credit risk component, assets	[2]	0	(1)	3	(3)
Total mark-to-market on financial instruments designated at fair value		0	(1)	3	(3)
Mark-to-market on related derivatives		0	0	0	0
Net realized gain on related long-term debt derivatives		0	0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		0	(1)	3	(3)
Gain (Loss) on Instruments Designated at Fair Value and Related Derivatives [Member] | Securities purchased under resale agreements [Member]
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Interest rate and other components	[1]	0	(1)	0	6
Credit risk component, assets	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		0	(1)	0	6
Mark-to-market on related derivatives		0	0	0	0
Net realized gain on related long-term debt derivatives		0	0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		0	(1)	0	6
Gain (Loss) on Instruments Designated at Fair Value and Related Derivatives [Member] | Securities sold under repurchase agreements [Member]
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Interest rate and other components	[1]	0	0	0	3
Credit risk component, liabilities	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		0	0	0	3
Mark-to-market on related derivatives		0	0	0	0
Net realized gain on related long-term debt derivatives		0	0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		0	0	0	3
Gain (Loss) on Instruments Designated at Fair Value and Related Derivatives [Member] | Long-term debt [Member]
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Interest rate and other components	[1]	51	0	173	(7)
Credit risk component, liabilities	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		51	0	173	(7)
Mark-to-market on related derivatives		(44)	(6)	(154)	(13)
Net realized gain on related long-term debt derivatives		10	16	34	45
Gain (loss) on instruments designated at fair value and related derivatives		17	10	53	25
Gain (Loss) on Instruments Designated at Fair Value and Related Derivatives [Member] | Hybrid Instrument [Member]
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Interest rate and other components	[1]	(229)	(355)	78	(922)
Credit risk component, liabilities	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		(229)	(355)	78	(922)
Mark-to-market on related derivatives		241	354	(71)	931
Net realized gain on related long-term debt derivatives		0	0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		$ 12	$ (1)	$ 7	$ 9

[1] As it relates to hybrid instruments, interest rate and other components primarily includes interest rate, foreign exchange and equity contract risks.

[2] The fair value movement on fair value option liabilities attributable to our own credit spread is recorded in common equity as a component of other comprehensive income (loss).