Derivative Financial Instruments - Gains and Losses on Hedged Items in Fair Value Hedges (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest Income Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	$ (99)	$ (109)	$ (146)
Gain (Loss) on Hedged Items	129	110	219
Other Income (Loss) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	396	9	(23)
Gain (Loss) on Hedged Items	(383)	(6)	6
Net Ineffective Gain (Loss) Recognized - Other Income	13	3	(17)
Available-for-sale securities [Member] | Interest Income [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(41)	(121)	(178)
Gain (Loss) on Hedged Items	362	367	374
Available-for-sale securities [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	322	33	59
Gain (Loss) on Hedged Items	(309)	(35)	(74)
Net Ineffective Gain (Loss) Recognized - Other Income	13	(2)	(15)
Long-term Debt [Member] | Interest Expense [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	(58)	12	32
Gain (Loss) on Hedged Items	(233)	(257)	(155)
Long-term Debt [Member] | Other Income (Loss) [Member] | Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative	74	(24)	(82)
Gain (Loss) on Hedged Items	(74)	29	80
Net Ineffective Gain (Loss) Recognized - Other Income	$ 0	$ 5	$ (2)