Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (24)	$ (32)	$ (53)	$ (55)
Gain (Loss) on Hedged Items	36	23	66	46
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	118	(115)	420	(42)
Gain (Loss) on Hedged Items	(113)	117	(409)	48
Net Ineffective Gain (Loss) Recognized - Other Income	5	2	11	6
Available-for-sale securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(10)	(36)	(33)	(67)
Gain (Loss) on Hedged Items	89	91	176	180
Available-for-sale securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	125	(147)	487	(61)
Gain (Loss) on Hedged Items	(117)	147	(474)	64
Net Ineffective Gain (Loss) Recognized - Other Income	8	0	13	3
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(14)	4	(20)	12
Gain (Loss) on Hedged Items	(53)	(68)	(110)	(134)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(7)	32	(67)	19
Gain (Loss) on Hedged Items	4	(30)	65	(16)
Net Ineffective Gain (Loss) Recognized - Other Income	$ (3)	$ 2	$ (2)	$ 3