Segment Reporting and Significant Customer Information - Revenue for Individual Countries that Exceed 10% of Total Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue, Major Customer [Line Items]
Total revenues	$ 30,825	$ 30,884
United States
Revenue, Major Customer [Line Items]
Total revenues	16,895	17,118
United Kingdom
Revenue, Major Customer [Line Items]
Total revenues	5,859	5,860
Germany
Revenue, Major Customer [Line Items]
Total revenues	3,641	3,659
Rest of the world
Revenue, Major Customer [Line Items]
Total revenues	$ 4,430	$ 4,247