Description of Business and Summary of Significant Accounting Policies - Trade Receivables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in Allowances for Uncollectible Accounts
Beginning balance	$ 43	$ 42	$ 43
Provision charged to expense	3	7	7
Write-offs, net of recoveries	(6)	(6)	(5)
Acquisitions and divestitures	0	1	0
Foreign currency translation	3	(1)	(3)
Ending balance	$ 43	$ 43	$ 42