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Investment Securities (Schedule of Unrealized Loss on Investments) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months $ 6,667 $ 12,356
Available-for-sale Securities, Unrealized Loss Less than 12 months (50) (317)
Available-for-sale Securities, Fair value 12 months or longer 8,358 2,038
Available-for-sale Securities, Unrealized Loss 12 months or longer (263) (118)
Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 6,541 12,205
Available-for-sale Securities, Unrealized Loss Less than 12 months (38) (309)
Available-for-sale Securities, Fair value 12 months or longer 8,242 1,769
Available-for-sale Securities, Unrealized Loss 12 months or longer (227) (72)
US Treasury and Government [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 471 1,612
Available-for-sale Securities, Unrealized Loss Less than 12 months (8) (60)
Available-for-sale Securities, Fair value 12 months or longer 1,305 0
Available-for-sale Securities, Unrealized Loss 12 months or longer (46) 0
US States and Political Subdivisions Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 242 524
Available-for-sale Securities, Unrealized Loss Less than 12 months (2) (19)
Available-for-sale Securities, Fair value 12 months or longer 183 0
Available-for-sale Securities, Unrealized Loss 12 months or longer (5) 0
Foreign Government Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 80 38
Available-for-sale Securities, Unrealized Loss Less than 12 months (1) 0
Available-for-sale Securities, Fair value 12 months or longer 4 0
Available-for-sale Securities, Unrealized Loss 12 months or longer 0 0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 4,066 8,052
Available-for-sale Securities, Unrealized Loss Less than 12 months (19) (196)
Available-for-sale Securities, Fair value 12 months or longer 5,671 587
Available-for-sale Securities, Unrealized Loss 12 months or longer (137) (16)
Residential Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 857 813
Available-for-sale Securities, Unrealized Loss Less than 12 months (2) (17)
Available-for-sale Securities, Fair value 12 months or longer 773 860
Available-for-sale Securities, Unrealized Loss 12 months or longer (32) (53)
Commercial Mortgage Backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 76 47
Available-for-sale Securities, Unrealized Loss Less than 12 months (1) (1)
Available-for-sale Securities, Fair value 12 months or longer 21 149
Available-for-sale Securities, Unrealized Loss 12 months or longer 0 (1)
Asset-backed Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 220 375
Available-for-sale Securities, Unrealized Loss Less than 12 months (1) (2)
Available-for-sale Securities, Fair value 12 months or longer 91 127
Available-for-sale Securities, Unrealized Loss 12 months or longer 0 0
Corporate Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 529 744
Available-for-sale Securities, Unrealized Loss Less than 12 months (4) (14)
Available-for-sale Securities, Fair value 12 months or longer 194 46
Available-for-sale Securities, Unrealized Loss 12 months or longer (7) (2)
Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Fair value Less than 12 months 126 151
Available-for-sale Securities, Unrealized Loss Less than 12 months (12) (8)
Available-for-sale Securities, Fair value 12 months or longer 116 269
Available-for-sale Securities, Unrealized Loss 12 months or longer $ (36) $ (46)