Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Securities Financing Transaction [Line Items]
Total	$ 3,407	$ 3,646
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,407	3,646
U.S. government and agency | Maturity Overnight
Securities Financing Transaction [Line Items]
Total	1,567	1,474
U.S. government and agency | Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	940	1,823
U.S. government and agency | Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	$ 900	$ 349