Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (29)	$ (23)
Gain (Loss) on Hedged Items	30	23
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	302	73
Gain (Loss) on Hedged Items	(296)	(69)
Net Ineffective Gain (Loss) Recognized - Other Income	6	4
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(23)	(31)
Gain (Loss) on Hedged Items	87	89
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	362	86
Gain (Loss) on Hedged Items	(357)	(83)
Net Ineffective Gain (Loss) Recognized - Other Income	5	3
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(6)	8
Gain (Loss) on Hedged Items	(57)	(66)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(60)	(13)
Gain (Loss) on Hedged Items	61	14
Net Ineffective Gain (Loss) Recognized - Other Income	$ 1	$ 1