v3.8.0.1
Investments (Repurchase Agreements and Securities Lending Transactions) (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase $ 8,400 $ 7,606
Total cash collateral for loaned securities 4,354 4,333
Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 912 956
Total cash collateral for loaned securities 4,319 4,259
Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 7,488 6,650
Total cash collateral for loaned securities 35 74
U.S. Treasury securities and obligations of U.S. government authorities and agencies    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 8,260 7,367
Total cash collateral for loaned securities 122 9
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 911 950
Total cash collateral for loaned securities 87 9
U.S. Treasury securities and obligations of U.S. government authorities and agencies | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 7,349 6,417
Total cash collateral for loaned securities 35 0
Obligations of U.S. states and their political subdivisions    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total cash collateral for loaned securities 103 18
Obligations of U.S. states and their political subdivisions | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total cash collateral for loaned securities 103 18
Obligations of U.S. states and their political subdivisions | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total cash collateral for loaned securities 0 0
Foreign government bonds    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total cash collateral for loaned securities 335 279
Foreign government bonds | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total cash collateral for loaned securities 335 279
Foreign government bonds | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total cash collateral for loaned securities 0 0
U.S. corporate public securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 1 0
Total cash collateral for loaned securities 2,961 2,731
U.S. corporate public securities | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 1 0
Total cash collateral for loaned securities 2,961 2,731
U.S. corporate public securities | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 0
Total cash collateral for loaned securities 0 0
Foreign corporate public securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 6
Total cash collateral for loaned securities 655 786
Foreign corporate public securities | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 6
Total cash collateral for loaned securities 655 786
Foreign corporate public securities | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 0
Total cash collateral for loaned securities 0 0
Residential mortgage-backed securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 139 233
Total cash collateral for loaned securities 0 129
Residential mortgage-backed securities | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 0
Total cash collateral for loaned securities 0 55
Residential mortgage-backed securities | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 139 233
Total cash collateral for loaned securities 0 74
Equity securities    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 0
Total cash collateral for loaned securities 178 381
Equity securities | Overnight & Continuous    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 0
Total cash collateral for loaned securities 178 381
Equity securities | Up to 30 Days    
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]    
Total securities sold under agreements to repurchase 0 0
Total cash collateral for loaned securities $ 0 $ 0