Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Securities Financing Transaction [Line Items]
Total	$ 30,384	$ 30,826
% of Total	100.00%	100.00%
U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	$ 27,247	$ 26,033
% of Total	89.70%	84.50%
Agency RMBS
Securities Financing Transaction [Line Items]
Total	$ 1,644	$ 3,054
% of Total	5.40%	9.90%
Foreign government
Securities Financing Transaction [Line Items]
Total	$ 1,197	$ 1,145
% of Total	3.90%	3.70%
U.S. corporate
Securities Financing Transaction [Line Items]
Total	$ 295	$ 307
% of Total	1.00%	1.00%
Foreign corporate
Securities Financing Transaction [Line Items]
Total	$ 1	$ 287
% of Total	0.00%	0.90%
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	$ 10,212	$ 10,701
Maturity Overnight | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	10,199	10,371
Maturity Overnight | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Overnight | Foreign government
Securities Financing Transaction [Line Items]
Total	2	30
Maturity Overnight | U.S. corporate
Securities Financing Transaction [Line Items]
Total	10	125
Maturity Overnight | Foreign corporate
Securities Financing Transaction [Line Items]
Total	1	175
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	13,465	12,233
Maturity Less than 30 Days | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	11,215	10,423
Maturity Less than 30 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	986	482
Maturity Less than 30 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	979	1,034
Maturity Less than 30 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	285	182
Maturity Less than 30 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	112
Maturity 30 to 90 Days
Securities Financing Transaction [Line Items]
Total	6,272	7,892
Maturity 30 to 90 Days | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	5,398	5,239
Maturity 30 to 90 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	658	2,572
Maturity 30 to 90 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	216	81
Maturity 30 to 90 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity 30 to 90 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Greater than 90 Days
Securities Financing Transaction [Line Items]
Total	435	0
Maturity Greater than 90 Days | U.S. Treasury and agency
Securities Financing Transaction [Line Items]
Total	435	0
Maturity Greater than 90 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Greater than 90 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Greater than 90 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Greater than 90 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0