Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 26,527	$ 26,755
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,400	102
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	25,024	25,136
U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,305	5
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	95	97
Foreign corporate
Securities Financing Transaction [Line Items]
Total	58	58
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	8,656	9,604
Maturity Less than 30 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,348	51
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,153	8,403
Maturity Less than 30 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,305	5
Maturity Less than 30 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	43	46
Maturity Less than 30 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	58	58
Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	11,623	10,543
Maturity 30 to 180 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	52	51
Maturity 30 to 180 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	11,623	10,125
Maturity 30 to 180 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Maturity 30 to 180 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	52	51
Maturity 30 to 180 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0