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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (30)	$ (44)	$ (110)	$ (138)
Gain (Loss) on Hedged Items	58	70	186	218
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(1)	(407)	(956)	(214)
Gain (Loss) on Hedged Items	(11)	389	895	204
Net Ineffective Gain (Loss) Recognized - Other Income	(12)	(18)	(61)	(10)
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(38)	(51)	(133)	(154)
Gain (Loss) on Hedged Items	92	92	281	273
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	26	(434)	(1,027)	(229)
Gain (Loss) on Hedged Items	(37)	414	965	217
Net Ineffective Gain (Loss) Recognized - Other Income	(11)	(20)	(62)	(12)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	8	7	23	16
Gain (Loss) on Hedged Items	(34)	(22)	(95)	(55)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(27)	27	71	15
Gain (Loss) on Hedged Items	26	(25)	(70)	(13)
Net Ineffective Gain (Loss) Recognized - Other Income	$ (1)	$ 2	$ 1	$ 2