Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (23)	$ (40)
Gain (Loss) on Hedged Items	23	63
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	73	(590)
Gain (Loss) on Hedged Items	(69)	561
Net Ineffective Gain (Loss) Recognized - Other Income	4	(29)
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(31)	(48)
Gain (Loss) on Hedged Items	89	93
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	86	(656)
Gain (Loss) on Hedged Items	(83)	626
Net Ineffective Gain (Loss) Recognized - Other Income	3	(30)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	8	8
Gain (Loss) on Hedged Items	(66)	(30)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(13)	66
Gain (Loss) on Hedged Items	14	(65)
Net Ineffective Gain (Loss) Recognized - Other Income	$ 1	$ 1