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Investment Securities - Investment Securities Available-for-Sale (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost $ 66,916 $ 68,717
Available-for-sale securities, Unrealized Gains 131 308
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (2,135) (888)
Available-for-sale securities, Fair Value [1] 64,912 68,137
U.S. Treasury and Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 20,176 23,586
Available-for-sale securities, Unrealized Gains 1 3
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (596) (288)
Available-for-sale securities, Fair Value 19,581 23,301
Residential Mortgage-Backed Securities [Member] | Agency [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 39,486 38,450
Available-for-sale securities, Unrealized Gains 103 152
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (1,279) (571)
Available-for-sale securities, Fair Value 38,310 38,031
Commercial [Member] | Agency [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 5 6
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Fair Value 5 6
Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 402  
Available-for-sale securities, Fair Value 408  
Asset-Backed Securities [Member] | Other Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 402 413
Available-for-sale securities, Unrealized Gains 6 6
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Fair Value 408 419
Obligations of State and Political Subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 6,847 6,240
Available-for-sale securities, Unrealized Gains 21 147
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (260) (29)
Available-for-sale securities, Fair Value 6,608 6,358
Other [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost   22
Available-for-sale securities, Unrealized Losses Other-than-Temporary $ 0 0
Available-for-sale securities, Fair Value   $ 22
[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.