Investments (Investment Income (Loss), Net) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Gains (losses) on sales of investments, net	$ 12	$ 46	$ 12
Investment impairment losses	(29)	(34)	(59)
Interest and dividend income	136	123	115
Other, net	134	78	13
Investment income (loss), net	$ 253	$ 213	$ 81