Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	$ (541)	$ (99)	$ (109)
Gain (Loss) on Hedged Items	[1]	383	129	110
Other Income (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	0	396	9
Gain (Loss) on Hedged Items	[1]	0	(383)	(6)
Net Ineffective Gain (Loss) Recognized - Other Income	[1]		13	3
Securities available-for-sale [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	(776)	(41)	(121)
Gain (Loss) on Hedged Items	[1]	1,086	362	367
Securities available-for-sale [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	0	322	33
Gain (Loss) on Hedged Items	[1]	0	(309)	(35)
Net Ineffective Gain (Loss) Recognized - Other Income	[1]		13	(2)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	235	(58)	12
Gain (Loss) on Hedged Items	[1]	(703)	(233)	(257)
Long-term Debt [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	[1]	0	74	(24)
Gain (Loss) on Hedged Items	[1]	$ 0	(74)	29
Net Ineffective Gain (Loss) Recognized - Other Income	[1]		$ 0	$ 5

[1] As a result of adopting new accounting guidance discussed in Note 2, "Summary of Significant Accounting Policies and New Accounting Pronouncements," 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.