Income Taxes (Schedule of Unrecognized Tax Benefits Rollforward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at January 1	$ (98)	$ (150)	$ (158)
(Additions) reductions based on tax positions related to current year	(54)	(4)	(6)
Additions for tax positions of prior years	(40)	(3)	(6)
Reductions for tax positions of prior years	4	33	7
Settlements	6	19	2
Expiration of statutes of limitations	1	5	4
Currency translation adjustment	(7)	2	7
Balance at December 31	$ (188)	$ (98)	$ (150)