Supplemental Financial Information - Components of Accounts Receivable (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Accounts Receivable, after Allowance for Credit Loss, Current [Abstract]
Accounts receivable, gross	$ 1,465	$ 1,387
Less: allowance for doubtful accounts	(35)	(31)
Total accounts receivable, net	1,430	1,356
Trade
Accounts Receivable, after Allowance for Credit Loss, Current [Abstract]
Accounts receivable, gross	1,435	1,364
Other
Accounts Receivable, after Allowance for Credit Loss, Current [Abstract]
Accounts receivable, gross	$ 30	$ 23