Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 3,646	$ 3,791
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,646	3,791
U.S. government and agency | Maturity Overnight
Securities Financing Transaction [Line Items]
Total	1,474	1,626
U.S. government and agency | Maturity less than 30 Days
Securities Financing Transaction [Line Items]
Total	1,823	964
U.S. government and agency | Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	$ 349	$ 1,201