Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (109)	$ (146)	$ (192)
Gain (Loss) on Hedged Items	110	219	280
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	9	(23)	(100)
Gain (Loss) on Hedged Items	(6)	6	86
Net Ineffective Gain (Loss) Recognized - Other Income	3	(17)	(14)
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(121)	(178)	(202)
Gain (Loss) on Hedged Items	367	374	363
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	33	59	(82)
Gain (Loss) on Hedged Items	(35)	(74)	66
Net Ineffective Gain (Loss) Recognized - Other Income	(2)	(15)	(16)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	12	32	10
Gain (Loss) on Hedged Items	(257)	(155)	(83)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(24)	(82)	(18)
Gain (Loss) on Hedged Items	29	80	20
Net Ineffective Gain (Loss) Recognized - Other Income	$ 5	$ (2)	$ 2