Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total	$ 32,119	$ 30,197
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	29,614	27,259
Agency RMBS
Securities Financing Transaction [Line Items]
Total	1,196	1,551
Foreign government
Securities Financing Transaction [Line Items]
Total	886	998
U.S. corporate
Securities Financing Transaction [Line Items]
Total	392	389
Foreign corporate
Securities Financing Transaction [Line Items]
Total	31	0
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	8,363	10,127
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	8,362	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	1	9
Maturity Overnight | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	11,420	12,998
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	10,162	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	836	510
Maturity Less than 30 Days | U.S. corporate
Securities Financing Transaction [Line Items]
Total	391	380
Maturity Less than 30 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	31	0
Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	12,336	7,072
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	11,090	5,986
Maturity 30 to 180 Days | Agency RMBS
Securities Financing Transaction [Line Items]
Total	1,196	600
Maturity 30 to 180 Days | Foreign government
Securities Financing Transaction [Line Items]
Total	50	486
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