Investments (Repurchase Agreements and Securities Lending Transactions) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	$ 9,681	$ 9,950
Total cash collateral for loaned securities	4,213	3,929
Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	9,681	9,779
Total cash collateral for loaned securities	4,213	3,929
Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	171
Total cash collateral for loaned securities	0	0
30 days or greater
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
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	9,431	9,589
Total cash collateral for loaned securities	9	105
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	9,431	9,418
Total cash collateral for loaned securities	9	105
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	0	171
Total cash collateral for loaned securities	0	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	33	88
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	33	88
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	244	325
Foreign government bonds | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	244	325
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. public corporate securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	19
Total cash collateral for loaned securities	2,996	2,563
U.S. public corporate securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total securities sold under agreements to repurchase	0	19
Total cash collateral for loaned securities	2,996	2,563
U.S. public corporate 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 public corporate securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	762	693
Foreign public corporate securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	762	693
Foreign public corporate securities | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	0	0
Commercial mortgage-backed securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	2	0
Commercial mortgage-backed securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	2	0
Commercial mortgage-backed securities | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
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	250	342
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	250	342
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	0	0
Equity securities
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	167	155
Equity securities | Overnight & Continuous
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	167	155
Equity securities | Up to 30 Days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Total cash collateral for loaned securities	$ 0	$ 0