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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (26)	$ (30)	$ (81)	$ (110)
Gain (Loss) on Hedged Items	25	58	71	186
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(3)	(1)	(45)	(956)
Gain (Loss) on Hedged Items	1	(11)	49	895
Net Ineffective Gain (Loss) Recognized - Other Income	(2)	(12)	4	(61)
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(28)	(38)	(95)	(133)
Gain (Loss) on Hedged Items	88	92	268	281
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	2	26	(59)	(1,027)
Gain (Loss) on Hedged Items	(2)	(37)	62	965
Net Ineffective Gain (Loss) Recognized - Other Income	0	(11)	3	(62)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	2	8	14	23
Gain (Loss) on Hedged Items	(63)	(34)	(197)	(95)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(5)	(27)	14	71
Gain (Loss) on Hedged Items	3	26	(13)	(70)
Net Ineffective Gain (Loss) Recognized - Other Income	$ (2)	$ (1)	$ 1	$ 1