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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (27)	$ (26)	$ (80)	$ (81)
Gain (Loss) on Hedged Items	39	25	105	71
Other Income (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	152	(3)	572	(45)
Gain (Loss) on Hedged Items	(147)	1	(556)	49
Net Ineffective Gain (Loss) Recognized - Other Income	5	(2)	16	4
Available-for-sale securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(9)	(28)	(42)	(95)
Gain (Loss) on Hedged Items	92	88	268	268
Available-for-sale securities [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	147	2	634	(59)
Gain (Loss) on Hedged Items	(142)	(2)	(616)	62
Net Ineffective Gain (Loss) Recognized - Other Income	5	0	18	3
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(18)	2	(38)	14
Gain (Loss) on Hedged Items	(53)	(63)	(163)	(197)
Long-term Debt [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	5	(5)	(62)	14
Gain (Loss) on Hedged Items	(5)	3	60	(13)
Net Ineffective Gain (Loss) Recognized - Other Income	$ 0	$ (2)	$ (2)	$ 1