Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 3,834	$ 3,791
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,834	3,791
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	1,345	1,626
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	1,596	964
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	$ 893	$ 1,201