Deferred Revenue (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Deferred revenue, advances received	$ 13,375
Deferred revenue	111,445	$ 118,414
Deferred revenue, current	$ 52,000	$ 36,100
Supply Agreement | Minimum
Deferred Revenue Arrangement [Line Items]
Deferred revenue, average initial contract term	1 year
Supply Agreement | Maximum
Deferred Revenue Arrangement [Line Items]
Deferred revenue, average initial contract term	5 years