Other Financial Data - Schedule of Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance at January 1	$ 56	$ 58	$ 72
Provision charged to Other general expenses	18	10	12
Accounts written off, net of recoveries	(18)	(15)	(32)
Foreign currency translation	3	3	6
Balance at December 31	$ 59	$ 56	$ 58