Derivative Financial Instruments - Gains and Losses on Derivative Instruments Designated and Qualifying As Hedging Instruments in Cash Flow Hedges (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in AOCI on Derivatives(1)	[1]	$ 84	$ (24)	$ 29
Net Interest Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified From AOCI into Income(1)	[1]	(11)	(35)	(23)
Foreign exchange contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in AOCI on Derivatives(1)	[1]	0	3	(2)
Foreign exchange contracts [Member] | Net Interest Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified From AOCI into Income(1)	[1]	0	0	0
Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in AOCI on Derivatives(1)	[1]	84	(27)	31
Interest rate contracts [Member] | Net Interest Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified From AOCI into Income(1)	[1]	$ (11)	$ (35)	$ (23)

[1] As a result of adopting new accounting guidance in 2019, beginning January 1, 2019, gains and losses on the derivatives in cash flow hedges are initially reported in AOCI and then reclassified into earnings in the same accounting period in which the designated forecasted transaction or hedged item affects earnings. Prior to January 1, 2019, cash flow hedge ineffectiveness was separately measured and reported immediately in other income. Cash flow hedge ineffectiveness was immaterial during 2018.