Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total	$ 32,006	$ 30,197
% of Total	100.00%	100.00%
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	$ 29,410	$ 27,259
% of Total	91.90%	90.30%
Agency RMBS
Securities Financing Transaction [Line Items]
Total	$ 1,265	$ 1,551
% of Total	4.00%	5.10%
Foreign government
Securities Financing Transaction [Line Items]
Total	$ 911	$ 998
% of Total	2.80%	3.30%
U.S. corporate
Securities Financing Transaction [Line Items]
Total	$ 408	$ 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	$ 8,386	$ 10,127
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	8,379	10,116
Maturity Overnight | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Overnight | Foreign government
Securities Financing Transaction [Line Items]
Total	0	2
Maturity Overnight | U.S. corporate
Securities Financing Transaction [Line Items]
Total	7	9
Maturity Overnight | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	10,841	12,998
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	9,517	11,157
Maturity Less than 30 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	0	951
Maturity Less than 30 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	911	510
Maturity Less than 30 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	401	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	12,779	7,072
Maturity 30 to 90 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	11,514	5,986
Maturity 30 to 90 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	1,265	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