Summary of Significant Accounting Policies - Deferred Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue recognized that was included in the contract liability balance	$ 12,900
Decrease in deferred revenue	(3,853)	$ (2,250)
Cash received in advance and not recognized as revenue	19,600
Accounting Standards Update 2014-09 | ASC 606 Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Decrease in deferred revenue	$ 2,800