Revenues - Contract Assets and Liabilities (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Contract Assets
Balance at December 31, 2018	$ 4
Increase due to revenue recognized during the period but contingent on future performance	8
Transferred to receivables from contract assets recognized at the beginning of the period	(8)
Balance at September 30, 2019	4
Contract Liabilities
Balance at December 31, 2018	64
Revenue recognized that was included in the deferred revenue balance at the beginning of the period	(50)
Increase due to consideration received, net of amount recognized as revenue during the period	31
Contract with Customer, Liability, Cumulative Catch-up Adjustment to Revenue, Modification of Contract	9
Balance at September 30, 2019	$ 54