Accounts Receivable, Net (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounts Receivable, Net
Advance cash payments received from third parties to mitigate credit risk	$ 136	$ 117
Standby letters of credit	$ 36	$ 54
Substantially all trade accounts receivable, net, maximum age of balances past their scheduled invoice date	30 days	30 days
Allowance for doubtful accounts receivable	$ 3	$ 3