Investments (Securities Lending Remaining Tenor) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total	$ 19,290	$ 19,417
U.S. government and agency
Securities Financing Transaction [Line Items]
Total	18,244	18,391
Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	903	1,026
Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	143	0
Maturity Overnight
Securities Financing Transaction [Line Items]
Total	3,056	3,753
Maturity Overnight | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	3,056	3,753
Maturity Overnight | Debt Security, Government, Non-US [Member]
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	7,377	6,223
Maturity Less than 30 Days | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,305	6,031
Maturity Less than 30 Days | Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	72	192
Maturity Less than 30 Days | Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	0	0
Maturity 30 to 180 Days [Member]
Securities Financing Transaction [Line Items]
Total	8,857	9,441
Maturity 30 to 180 Days [Member] | U.S. government and agency
Securities Financing Transaction [Line Items]
Total	7,883	8,607
Maturity 30 to 180 Days [Member] | Debt Security, Government, Non-US [Member]
Securities Financing Transaction [Line Items]
Total	831	834
Maturity 30 to 180 Days [Member] | Agency Securities [Member]
Securities Financing Transaction [Line Items]
Total	$ 143	$ 0