Investments (Repurchase Agreements - Securities Lending) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 19,417	$ 20,114
Securities Lending Reinvestment Portfolio Estimated Fair Value	19,508	20,133
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,102	102
Securities Lending Reinvestment Portfolio Estimated Fair Value	1,102	100
Estimated fair value | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Securities Sold under Agreements to Repurchase	1,141	113
Amortized cost | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Securities Sold under Agreements to Repurchase	$ 994	$ 98