Fair Value Option - Components of Gain (Loss) on Instruments at Fair Value and Related Derivatives (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Mark-to-market on related derivatives		$ 105	$ 227	$ (402)	$ 567
Gain (loss) on instruments designated at fair value and related derivatives		4	(1)	34	33
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]	(100)	(227)	429	(564)
Credit risk component, total	[2]	(1)	(1)	3	(2)
Total mark-to-market on financial instruments designated at fair value		(101)	(228)	432	(566)
Mark-to-market on related derivatives		94	213	(422)	570
Net realized gain on related long-term debt derivatives		11	14	24	29
Gain (loss) on instruments designated at fair value and related derivatives		4	(1)	34	33
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]	(1)	(1)	3	(2)
Total mark-to-market on financial instruments designated at fair value		(1)	(1)	3	(2)
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		(1)	(1)	3	(2)
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	7
Credit risk component, assets	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		0	1	0	7
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	7
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]	1	2	0	3
Credit risk component, liabilities	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		1	2	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		1	2	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]	34	(26)	122	(7)
Credit risk component, liabilities	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		34	(26)	122	(7)
Mark-to-market on related derivatives		(33)	17	(110)	(7)
Net realized gain on related long-term debt derivatives		11	14	24	29
Gain (loss) on instruments designated at fair value and related derivatives		12	5	36	15
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]	(135)	(204)	307	(567)
Credit risk component, liabilities	[2]	0	0	0	0
Total mark-to-market on financial instruments designated at fair value		(135)	(204)	307	(567)
Mark-to-market on related derivatives		127	196	(312)	577
Net realized gain on related long-term debt derivatives		0	0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		$ (8)	$ (8)	$ (5)	$ 10

[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.