Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (173)	$ (29)
Gain (Loss) on Hedged Items	151	30
Other Income (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	0	302
Gain (Loss) on Hedged Items	0	(296)
Net Ineffective Gain (Loss) Recognized - Other Income	0	6
Securities available-for-sale [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(257)	(23)
Gain (Loss) on Hedged Items	350	87
Securities available-for-sale [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	0	362
Gain (Loss) on Hedged Items	0	(357)
Net Ineffective Gain (Loss) Recognized - Other Income	0	5
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	84	(6)
Gain (Loss) on Hedged Items	(199)	(57)
Long-term Debt [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	0	(60)
Gain (Loss) on Hedged Items	0	61
Net Ineffective Gain (Loss) Recognized - Other Income	$ 0	$ 1