Revenue - Deferred Revenue (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Contract Liability [Roll Forward]
Beginning Balance, January 1, 2018	$ 56
Increases due to cash received or receivable, excluding amounts recognized as revenue during the period	14
Revenue recognized that was included in the deferred revenue balance at the beginning of the period	(16)
Ending Balance, March 31, 2018	$ 54