Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total	$ 30,986	$ 30,197
% of Total	100.00%	100.00%
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	$ 28,494	$ 27,259
% of Total	92.00%	90.30%
Agency RMBS
Securities Financing Transaction [Line Items]
Total	$ 1,277	$ 1,551
% of Total	4.10%	5.10%
Foreign government
Securities Financing Transaction [Line Items]
Total	$ 805	$ 998
% of Total	2.60%	3.30%
U.S. corporate
Securities Financing Transaction [Line Items]
Total	$ 398	$ 389
% of Total	1.30%	1.30%
Foreign corporate
Securities Financing Transaction [Line Items]
Total	$ 12	$ 0
% of Total	0.00%	0.00%
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	$ 8,690	$ 10,127
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	8,665	10,116
Maturity Overnight | Agency RMBS
Securities Financing Transaction [Line Items]
Total	12	0
Maturity Overnight | Foreign government
Securities Financing Transaction [Line Items]
Total	2	2
Maturity Overnight | U.S. corporate
Securities Financing Transaction [Line Items]
Total	11	9
Maturity Overnight | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	12,220	12,998
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	10,353	11,157
Maturity Less than 30 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	665	951
Maturity Less than 30 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	803	510
Maturity Less than 30 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	387	380
Maturity Less than 30 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	12	0
Maturity 30 to 90 Days
Securities Financing Transaction [Line Items]
Total	10,076	7,072
Maturity 30 to 90 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	9,476	5,986
Maturity 30 to 90 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	600	600
Maturity 30 to 90 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	0	486
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