Restructuring Charges/Other - Components of Restructuring Charges and Other (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring And Related Activities [Abstract]
Restructuring charges	[1]									$ 11	$ 25	$ 14
Gains on sales of assets										0	(40)	(83)
Other										0	0	(2)
Restructuring charges/other		$ 3	$ 1	$ 3	$ 4	$ (20)	$ 1	$ 2	$ 2	$ 11	$ (15)	$ (71)

[1] Includes severance and benefits, accelerated depreciation, changes in estimates or other exit costs.