Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 19,996	$ 20,114
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	19,008	19,359
Foreign government
Securities Financing Transaction [Line Items]
Total	988	712
U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	43
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	4,362	4,480
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	4,362	4,480
Maturity Overnight | Foreign government
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Overnight | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	8,459	7,108
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,952	6,496
Maturity Less than 30 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	507	569
Maturity Less than 30 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	43
Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	7,175	8,526
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,694	8,383
Maturity 30 to 180 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	481	143
Maturity 30 to 180 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0