Basis of Presentation and Significant Accounting Policies - Accounts Receivable and Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	$ 42	$ 26	$ 17
Provision charged to expense	36	24	15
Write-offs, less recoveries, and other adjustments	(26)	(8)	(6)
Ending balance	$ 52	$ 42	$ 26