Investments (Current and Long-term Equity Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Investment [Line Items]
Equity Securities	$ 2,306	$ 3,632
Exchange Traded Funds [Member]
Investment [Line Items]
Equity Securities	487	1,300
Fixed Maturity Mutual Funds [Member]
Investment [Line Items]
Equity Securities	611	791
Common Equity Securities [Member]
Investment [Line Items]
Equity Securities	890	1,254
Private Equity Securities [Member]
Investment [Line Items]
Equity Securities	$ 318	$ 287