Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Interest Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	$ (418)	$ (541)	$ (99)
Gain (Loss) on Hedged Items	[1]	378	383	129
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]			396
Gain (Loss) on Hedged Items	[1]			(383)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	[1]			13
Securities available-for-sale [Member] | Net Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	(731)	(776)	(41)
Gain (Loss) on Hedged Items	[1]	900	1,086	362
Securities available-for-sale [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]			322
Gain (Loss) on Hedged Items	[1]			(309)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	[1]			13
Deposits [Member] | Net Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	148
Gain (Loss) on Hedged Items	[1]	(261)
Long-term Debt [Member] | Net Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	165	235	(58)
Gain (Loss) on Hedged Items	[1]	$ (261)	$ (703)	(233)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]			74
Gain (Loss) on Hedged Items	[1]			(74)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	[1]			$ 0

[1] As a result of adopting new accounting guidance in 2019, beginning January 1, 2019, we report gains and losses on the derivatives and the hedged items in fair value hedges in net interest income. Prior to January 1, 2019, fair value hedge ineffectiveness was separately measured and reported in other income.