Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 19,244	$ 19,417
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	18,207	18,391
Foreign government
Securities Financing Transaction [Line Items]
Total	962	1,026
Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	75	0
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	4,042	3,753
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	4,042	3,753
Maturity Overnight | Foreign government
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Overnight | Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Maturity Less than 30 Days
Securities Financing Transaction [Line Items]
Total	4,594	6,223
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 | Foreign government
Securities Financing Transaction [Line Items]
Total	245	192
Maturity Less than 30 Days | Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	75	0
Maturity 30 to 180 Days [Member]
Securities Financing Transaction [Line Items]
Total	10,608	9,441
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] | Foreign government
Securities Financing Transaction [Line Items]
Total	717	834
Maturity 30 to 180 Days [Member] | Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	$ 0	$ 0