Revenues - Contract Assets and Liabilities (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Contract Assets
Balance at December 31, 2018	$ 4
Increase due to revenue recognized during the period but contingent on future performance	10
Transferred to receivables from contract assets recognized at the beginning of the period	(2)
Balance at March 31, 2019	12
Contract Liabilities
Balance at December 31, 2018	64
Revenue recognized that was included in the deferred revenue balance at the beginning of the period	(83)
Increase due to cash received, excluding amount recognized as revenue during the period	75
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	3
Balance at March 31, 2019	$ 59