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Investment Securities - Investment Securities Available-for-Sale (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost $ 67,930 $ 68,717
Available-for-sale securities, Unrealized Gains 181 308
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (1,764) (888)
Available-for-sale securities, Fair Value [1] 66,347 68,137
U.S. Treasury and Agencies [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 21,244 23,586
Available-for-sale securities, Unrealized Gains 2 3
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (515) (288)
Available-for-sale securities, Fair Value 20,731 23,301
Residential Mortgage-Backed Securities [Member] | Agency [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 39,409 38,450
Available-for-sale securities, Unrealized Gains 124 152
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (1,113) (571)
Available-for-sale securities, Fair Value 38,420 38,031
Commercial [Member] | Agency [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 6 6
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Fair Value 6 6
Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 405  
Available-for-sale securities, Fair Value 411  
Asset-Backed Securities [Member] | Other Asset-Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 405 413
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 411 419
Obligations of State and Political Subdivisions [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale securities, Amortized Cost 6,866 6,240
Available-for-sale securities, Unrealized Gains 49 147
Available-for-sale securities, Unrealized Losses Other-than-Temporary 0 0
Available-for-sale securities, Unrealized Losses Other (136) (29)
Available-for-sale securities, Fair Value 6,779 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.