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Revenue - Disaggregated by Timing of Recognition (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Disaggregation of Revenue        
Revenue $ 9,178 $ 11,518 $ 27,850 $ 37,136
Point in time        
Disaggregation of Revenue        
Revenue 8,525 10,613 26,156 34,729
Over time        
Disaggregation of Revenue        
Revenue $ 653 $ 905 $ 1,694 $ 2,407