Noncurrent Other Liabilities (Schedule of Changes in Asset Retirement Obligations) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Liabilities, Noncurrent [Roll Forward]
Balance at beginning of year		$ 26	$ 29	$ 36
Additions	[1]	2	0	2
Accretion		0	1	1
Payments		(4)	(5)	(10)
Revisions to cash flow estimates	[2]	(8)	1	0
Balance at end of year		$ 16	$ 26	$ 29

[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.