Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 18,111	$ 19,417
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	17,044	18,391
Foreign government
Securities Financing Transaction [Line Items]
Total	1,067	1,026
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	3,493	3,753
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,493	3,753
Maturity Overnight | Foreign government
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	6,594	6,223
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,282	6,031
Maturity Less than 30 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	312	192
Maturity 30 to 180 Days [Member]
Securities Financing Transaction [Line Items]
Total	8,024	9,441
Maturity 30 to 180 Days [Member] | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,269	8,607
Maturity 30 to 180 Days [Member] | Foreign government
Securities Financing Transaction [Line Items]
Total	$ 755	$ 834