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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