Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (276)	$ (27)	$ (700)	$ (80)
Gain (Loss) on Hedged Items	242	39	591	105
Other Income (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	0	152	0	572
Gain (Loss) on Hedged Items	0	(147)	0	(556)
Net Ineffective Gain (Loss) Recognized - Other Income	0	5	0	16
Securities available-for-sale [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(327)	(9)	(988)	(42)
Gain (Loss) on Hedged Items	415	92	1,253	268
Securities available-for-sale [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	0	147	0	634
Gain (Loss) on Hedged Items	0	(142)	0	(616)
Net Ineffective Gain (Loss) Recognized - Other Income	0	5	0	18
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	51	(18)	288	(38)
Gain (Loss) on Hedged Items	(173)	(53)	(662)	(163)
Long-term Debt [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	0	5	0	(62)
Gain (Loss) on Hedged Items	0	(5)	0	60
Net Ineffective Gain (Loss) Recognized - Other Income	$ 0	$ 0	$ 0	$ (2)