Income Taxes (Source of Income (Loss) Before Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income from continuing operations before income taxes	$ 949	$ 1,590	$ 1,568
Income from discontinued operations before income taxes	0	1	15
Income before income tax	$ 949	$ 1,591	$ 1,583