Investments (Securities Lending Programs) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	$ 564	$ 604
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	564
Overnight and Continuous [Member] | United States Government Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	42
Overnight and Continuous [Member] | Corporate Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	474
Overnight and Continuous [Member] | Equity Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	$ 48