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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (40)	$ (43)	$ (80)	$ (94)
Gain (Loss) on Hedged Items	65	71	128	148
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(365)	441	(955)	193
Gain (Loss) on Hedged Items	345	(425)	906	(185)
Net Ineffective Gain (Loss) Recognized - Other Income	(20)	16	(49)	8
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(47)	(49)	(95)	(103)
Gain (Loss) on Hedged Items	96	93	189	181
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(397)	453	(1,053)	205
Gain (Loss) on Hedged Items	376	(437)	1,002	(197)
Net Ineffective Gain (Loss) Recognized - Other Income	(21)	16	(51)	8
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	7	6	15	9
Gain (Loss) on Hedged Items	(31)	(22)	(61)	(33)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	32	(12)	98	(12)
Gain (Loss) on Hedged Items	(31)	12	(96)	12
Net Ineffective Gain (Loss) Recognized - Other Income	$ 1	$ 0	$ 2	$ 0