Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total	$ 26,440	$ 26,755
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,951	102
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	25,490	25,136
U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,855	5
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	96	97
Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	58
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	13,460	9,604
Maturity Less than 30 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,899	51
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	12,510	8,403
Maturity Less than 30 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	1,855	5
Maturity Less than 30 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	44	46
Maturity Less than 30 Days | Foreign corporate
Securities Financing Transaction [Line Items]
Total	0	58
Maturity 30 to 180 Days
Securities Financing Transaction [Line Items]
Total	6,783	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	6,783	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