Revenues - Contract Assets and Liabilities (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Contract Assets
Balance, as of January 1, 2018	$ 10
Increase due to revenue recognized during the period but contingent on future performance	6
Transferred to receivables from contract assets recognized at the beginning of the period	(1)
Balance, as of March 31, 2018	15
Contract Liabilities
Balance, as of January 1, 2018	42
Revenue recognized that was included in the deferred revenue balance at the beginning of the period	(20)
Increase due to cash received, excluding amount recognized as revenue during the period	25
Balance, as of March 31, 2018	$ 47