Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings	$ (74,738)		$ (74,738)			$ (71,453)
Deferred revenue	700		700
Deferred expense	300		300		$ 400
Operating expenses	(50,374)	$ (44,915)	(138,332)	$ (126,332)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to retained earnings					300
Deferred revenue					$ 700
Operating expenses	$ (3,468)		$ (10,092)