Investments (Repurchase Agreements - Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 19,244	$ 19,417
Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,560	1,102
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	18,207	18,391
U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,465	1,005
Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	95	97
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	4,594	6,223
Maturity Less than 30 Days | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,509	1,049
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	4,274	6,031
Maturity Less than 30 Days | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	2,465	1,005
Maturity Less than 30 Days | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	44	44
Maturity 30 to 180 Days [Member]
Securities Financing Transaction [Line Items]
Total	10,608	9,441
Maturity 30 to 180 Days [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	51	53
Maturity 30 to 180 Days [Member] | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	9,891	8,607
Maturity 30 to 180 Days [Member] | U.S. government and agency | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Maturity 30 to 180 Days [Member] | Other Debt Obligations [Member] | Repurchase Agreements [Member]
Securities Financing Transaction [Line Items]
Total	$ 51	$ 53