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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, 2018
Mar. 31, 2017
Revenue, Major Customer [Line Items]    
Total revenues $ 30,884 $ 28,429
United States    
Revenue, Major Customer [Line Items]    
Total revenues 17,118 16,226
United Kingdom    
Revenue, Major Customer [Line Items]    
Total revenues 5,860 4,950
GERMANY    
Revenue, Major Customer [Line Items]    
Total revenues 3,659 3,506
Rest of the world    
Revenue, Major Customer [Line Items]    
Total revenues $ 4,247 $ 3,747