Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 19,417	$ 20,114
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	18,391	19,359
Foreign government
Securities Financing Transaction [Line Items]
Total	1,026	712
U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	43
Open
Securities Financing Transaction [Line Items]
Total	3,753	4,480
Open | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,753	4,480
Open | Foreign government
Securities Financing Transaction [Line Items]
Total	0	0
Open | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	0
1 Month or Less
Securities Financing Transaction [Line Items]
Total	6,223	7,108
1 Month or Less | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,031	6,496
1 Month or Less | Foreign government
Securities Financing Transaction [Line Items]
Total	192	569
1 Month or Less | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	43
Over 1 to 6 Months
Securities Financing Transaction [Line Items]
Total	9,441	8,526
Over 1 to 6 Months | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	8,607	8,383
Over 1 to 6 Months | Foreign government
Securities Financing Transaction [Line Items]
Total	834	143
Over 1 to 6 Months | U.S. corporate
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0