Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 26,440	$ 26,755
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	25,490	25,136
Agency RMBS
Securities Financing Transaction [Line Items]
Total	256	274
Foreign government
Securities Financing Transaction [Line Items]
Total	694	764
U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	523
Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	58
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	6,197	6,608
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,197	6,608
Maturity Overnight | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	0
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 Overnight | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	13,460	9,604
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	12,510	8,403
Maturity Less than 30 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	256	0
Maturity Less than 30 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	694	620
Maturity Less than 30 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	523
Maturity Less than 30 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	58
Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	6,783	10,543
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	6,783	10,125
Maturity 30 to 180 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	274
Maturity 30 to 180 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	0	144
Maturity 30 to 180 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity 30 to 180 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0