Investments (Securities Lending Programs) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	$ 741	$ 454
Overnight and Continuous [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Collateral received for securities loaned, at carrying value	741
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	131
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	456
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	$ 154