Revenue, Deferred Revenue and Deferred Commissions - Deferred Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Contract With Customer, Asset And Liability [Roll Forward]
Beginning balance	$ 29,920
Additions	(1,045)	$ 2,579	$ 2,110
Ending balance	26,761	29,920
Accounting Standards Update 2014-09
Contract With Customer, Asset And Liability [Roll Forward]
Beginning balance	2,600
Additions	67,914
Revenue Recognized	(71,073)
Ending balance		2,600
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Contract With Customer, Asset And Liability [Roll Forward]
Beginning balance	(4,600)
Ending balance	$ (4,600)	$ (4,600)