Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 3,777	$ 3,791
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,777	3,791
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	1,253	1,626
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	1,414	964
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	$ 1,110	$ 1,201