Fair Value Option - Components of Gain (Loss) on Instruments at Fair Value and Related Derivatives (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components Of Gain On Instruments At Fair Value And Related Derivatives [Line Items]
Mark-to-market on the related derivatives		$ 1,136	$ 414	$ 9
Gain (loss) on instruments designated at fair value and related derivatives		43	(71)	264
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]	(1,144)	(541)	98
Credit risk component, total	[2]	(2)	(14)	200
Total mark-to-market on financial instruments designated at fair value		(1,146)	(555)	298
Mark-to-market on the related derivatives		1,132	421	(102)
Net realized gain on the related long-term debt derivatives		57	63	68
Gain (loss) on instruments designated at fair value and related derivatives		43	(71)	264
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
Credit risk component, assets	[2]	(2)	22	(14)
Total mark-to-market on financial instruments designated at fair value		(2)	22	(14)
Mark-to-market on the related derivatives		0	0	0
Net realized gain on the related long-term debt derivatives		0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		(2)	22	(14)
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]	(1)	0	0
Credit risk component, assets	[2]	0	0	0
Total mark-to-market on financial instruments designated at fair value		(1)	0	0
Mark-to-market on the related derivatives		0	0	0
Net realized gain on the related long-term debt derivatives		0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		(1)	0	0
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]	2	4	(6)
Credit risk component, liabilities	[2]	0	0	0
Total mark-to-market on financial instruments designated at fair value		2	4	(6)
Mark-to-market on the related derivatives		0	0	0
Net realized gain on the related long-term debt derivatives		0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		2	4	(6)
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]	6	(2)	(23)
Credit risk component, liabilities	[2]	0	(3)	194
Total mark-to-market on financial instruments designated at fair value		6	(5)	171
Mark-to-market on the related derivatives		(31)	(31)	7
Net realized gain on the related long-term debt derivatives		57	63	68
Gain (loss) on instruments designated at fair value and related derivatives		32	27	246
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]	(1,151)	(543)	127
Credit risk component, liabilities	[2]	0	(33)	20
Total mark-to-market on financial instruments designated at fair value		(1,151)	(576)	147
Mark-to-market on the related derivatives		1,163	452	(109)
Net realized gain on the related long-term debt derivatives		0	0	0
Gain (loss) on instruments designated at fair value and related derivatives		$ 12	$ (124)	$ 38

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

[2] As discussed more fully in Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," beginning January 1, 2017, 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).