Restructuring Charges/Other - Components of Restructuring Charges and Other Included in Other for Segment Reporting Purposes (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restructuring And Related Activities [Abstract]
Restructuring charges	[1]									$ 6	$ 11	$ 25
Gains on sales of assets										(3)	0	(40)
Restructuring charges/other		$ (2)	$ 1	$ 3	$ 1	$ 3	$ 1	$ 3	$ 4	$ 3	$ 11	$ (15)

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