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Information about Geographic Areas - Narrative (Details)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
segment
Jun. 30, 2017
USD ($)
Concentration Risk [Line Items]        
Number of reporting segments (segment) | segment     1  
Total revenue $ 26,428 $ 23,912 $ 51,676 $ 49,445
Revenue from Contract with Customer | China        
Concentration Risk [Line Items]        
Concentration risk percent 9.00% 12.00% 11.00% 11.00%
Total revenue $ 2,300 $ 2,800    
Revenue from Contract with Customer | United Kingdom        
Concentration Risk [Line Items]        
Concentration risk percent 10.00% 12.00% 10.00% 10.00%