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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (32)	$ (40)	$ (55)	$ (80)
Gain (Loss) on Hedged Items	23	65	46	128
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(115)	(365)	(42)	(955)
Gain (Loss) on Hedged Items	117	345	48	906
Net Ineffective Gain (Loss) Recognized - Other Income	2	(20)	6	(49)
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(36)	(47)	(67)	(95)
Gain (Loss) on Hedged Items	91	96	180	189
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(147)	(397)	(61)	(1,053)
Gain (Loss) on Hedged Items	147	376	64	1,002
Net Ineffective Gain (Loss) Recognized - Other Income	0	(21)	3	(51)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	4	7	12	15
Gain (Loss) on Hedged Items	(68)	(31)	(134)	(61)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	32	32	19	98
Gain (Loss) on Hedged Items	(30)	(31)	(16)	(96)
Net Ineffective Gain (Loss) Recognized - Other Income	$ 2	$ 1	$ 3	$ 2