Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total	$ 26,755	$ 30,197
U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	25,136	27,259
Agency RMBS
Securities Financing Transaction [Line Items]
Total	274	1,551
Foreign government
Securities Financing Transaction [Line Items]
Total	764	998
U.S. corporate
Securities Financing Transaction [Line Items]
Total	523	389
Foreign corporate
Securities Financing Transaction [Line Items]
Total	58	0
Open
Securities Financing Transaction [Line Items]
Total	6,608	10,127
Open | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	6,608	10,116
Open | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	0
Open | Foreign government
Securities Financing Transaction [Line Items]
Total	0	2
Open | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	9
Open | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	0
1 Month or Less
Securities Financing Transaction [Line Items]
Total	9,604	12,998
1 Month or Less | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	8,403	11,157
1 Month or Less | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	951
1 Month or Less | Foreign government
Securities Financing Transaction [Line Items]
Total	620	510
1 Month or Less | U.S. corporate
Securities Financing Transaction [Line Items]
Total	523	380
1 Month or Less | Foreign corporate
Securities Financing Transaction [Line Items]
Total	58	0
1 to 6 Months
Securities Financing Transaction [Line Items]
Total	10,543	7,072
1 to 6 Months | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	10,125	5,986
1 to 6 Months | Agency RMBS
Securities Financing Transaction [Line Items]
Total	274	600
1 to 6 Months | Foreign government
Securities Financing Transaction [Line Items]
Total	144	486
1 to 6 Months | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	0
1 to 6 Months | Foreign corporate
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0