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Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities $ 30,981 $ 37,502
Unrealized Gains, Available-for-sale securities 263 229
Unrealized Losses, Available-for-sale securities (544) (821)
Fair Value, Available-for-sale securities 30,700 36,910
Amortized Cost, Held-to-Maturity Securities 13,977 12,809
Unrealized Gains, Held-to-maturity securities 74 116
Unrealized Losses, Held-to-maturity securities (149) (87)
Held-to-maturity Securities, Fair Value 13,902 12,838
U.S. Treasury [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 15,340 21,366
Unrealized Gains, Available-for-sale securities 153 102
Unrealized Losses, Available-for-sale securities (329) (559)
Fair Value, Available-for-sale securities 15,164 20,909
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 3,616 4,535
Unrealized Gains, Available-for-sale securities 0 4
Unrealized Losses, Available-for-sale securities (77) (122)
Fair Value, Available-for-sale securities 3,539 4,417
Amortized Cost, Held-to-Maturity Securities 2,113 2,465
Unrealized Gains, Held-to-maturity securities 10 11
Unrealized Losses, Held-to-maturity securities (12) (9)
Held-to-maturity Securities, Fair Value 2,111 2,467
Collateralized Mortgage Obligations, Issued by U.S. Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 648 2,139
Unrealized Gains, Available-for-sale securities 0 0
Unrealized Losses, Available-for-sale securities (24) (36)
Fair Value, Available-for-sale securities 624 2,103
Amortized Cost, Held-to-Maturity Securities 1,281 1,591
Unrealized Gains, Held-to-maturity securities 41 59
Unrealized Losses, Held-to-maturity securities (18) (12)
Held-to-maturity Securities, Fair Value 1,304 1,638
Direct Agency Obligations U S Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 3,369 3,709
Unrealized Gains, Available-for-sale securities 85 118
Unrealized Losses, Available-for-sale securities 0 (10)
Fair Value, Available-for-sale securities 3,454 3,817
Mortgage-backed Securities, Issued or Guaranteed by US Government Agency [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 5,152 3,102
Unrealized Gains, Available-for-sale securities 1 2
Unrealized Losses, Available-for-sale securities (87) (58)
Fair Value, Available-for-sale securities 5,066 3,046
Amortized Cost, Held-to-Maturity Securities 2,485 2,557
Unrealized Gains, Held-to-maturity securities 4 11
Unrealized Losses, Held-to-maturity securities (13) (10)
Held-to-maturity Securities, Fair Value 2,476 2,558
Collateralized Mortgage Obligations, Issued or Guaranteed by US Government Agency [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 792 1,370
Unrealized Gains, Available-for-sale securities 1 2
Unrealized Losses, Available-for-sale securities (18) (15)
Fair Value, Available-for-sale securities 775 1,357
Amortized Cost, Held-to-Maturity Securities 8,083 6,176
Unrealized Gains, Held-to-maturity securities 18 34
Unrealized Losses, Held-to-maturity securities (106) (56)
Held-to-maturity Securities, Fair Value 7,995 6,154
Direct Agency Obligations [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 419 423
Unrealized Gains, Available-for-sale securities 19 1
Unrealized Losses, Available-for-sale securities 0 (4)
Fair Value, Available-for-sale securities 438 420
Obligations of U.S. states and political subdivision [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Held-to-Maturity Securities 12 15
Unrealized Gains, Held-to-maturity securities 1 1
Unrealized Losses, Held-to-maturity securities 0 0
Held-to-maturity Securities, Fair Value 13 16
Home Equity [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 54 69
Unrealized Gains, Available-for-sale securities 0 0
Unrealized Losses, Available-for-sale securities (3) (8)
Fair Value, Available-for-sale securities 51 61
Other [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 506 108
Unrealized Gains, Available-for-sale securities 4 0
Unrealized Losses, Available-for-sale securities 0 (3)
Fair Value, Available-for-sale securities 510 105
Foreign debt securities [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities [1] 902 522
Unrealized Gains, Available-for-sale securities [1] 0 0
Unrealized Losses, Available-for-sale securities [1] 0 (1)
Fair Value, Available-for-sale securities [1] 902 521
Equity securities [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Available-for-sale securities 183 159
Unrealized Gains, Available-for-sale securities 0 0
Unrealized Losses, Available-for-sale securities (6) (5)
Fair Value, Available-for-sale securities 177 154
Residential Mortgages [Member]    
Schedule of Available-for-sale and Held-to-Maturity Securities [Line Items]    
Amortized Cost, Held-to-Maturity Securities 3 5
Unrealized Gains, Held-to-maturity securities 0 0
Unrealized Losses, Held-to-maturity securities 0 0
Held-to-maturity Securities, Fair Value $ 3 $ 5
[1] Foreign debt securities represent public sector entity, bank or corporate debt.