Noncurrent Other Liabilities (Schedule of Changes in Asset Retirement Obligations) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Liabilities, Noncurrent [Abstract]
Balance at beginning of year		$ 29	$ 36	$ 37
Additions	[1]	0	2	0
Accretion		1	1	1
Payments		(5)	(10)	(4)
Revisions to cash flow estimates	[2]	1	0	2
Balance at end of year		$ 26	$ 29	$ 36

[1] Primarily relates to sites which management no longer considers to have an indeterminate life.

[2] Primarily relates to revisions to the estimated cost and timing of future obligations.