Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (146)	$ (192)	$ (237)
Gain (Loss) on Hedged Items	219	280	353
Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(23)	(100)	(688)
Gain (Loss) on Hedged Items	6	86	672
Net Ineffective Gain (Loss) Recognized - Other Income	(17)	(14)	(16)
Available-for-sale Securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(178)	(202)	(246)
Gain (Loss) on Hedged Items	374	363	375
Available-for-sale Securities [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	59	(82)	(684)
Gain (Loss) on Hedged Items	(74)	66	668
Net Ineffective Gain (Loss) Recognized - Other Income	(15)	(16)	(16)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	32	10	9
Gain (Loss) on Hedged Items	(155)	(83)	(22)
Long-term Debt [Member] | Other Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(82)	(18)	(4)
Gain (Loss) on Hedged Items	80	20	4
Net Ineffective Gain (Loss) Recognized - Other Income	$ (2)	$ 2	$ 0