Revenue Recognition - Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Balance as of December 31, 2017	$ 51
Provision for doubtful accounts	7
Write-offs less recoveries	(14)
Balance as of September 30, 2018	44
Allowance for Trade Receivables [Member]
Contract with Customer, Asset, Accumulated Allowance for Credit Loss	41	$ 44
Provision for doubtful accounts	9
Write-offs less recoveries	(12)
Trade Accounts Receivable [Member]
Balance as of December 31, 2017	7
Provision for doubtful accounts	(2)
Write-offs less recoveries	(2)
Balance as of September 30, 2018	$ 3