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Revenue (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Disaggregation of Revenue [Line Items]          
Deferred service revenue, current $ 6,379   $ 6,379   $ 6,537
Deferred service revenue, non-current 1,435   1,435   890
Revenue 21,915 $ 18,160 62,961 $ 59,100  
Prepaid expenses and other current assets 2,704   2,704   $ 2,832
Service [Member]          
Disaggregation of Revenue [Line Items]          
Deferred service revenue 7,800   7,800    
Deferred service revenue, current 6,400   6,400    
Deferred service revenue, non-current 1,400   1,400    
Revenue 1,300   5,700    
Prepaid expenses and other current assets $ 400   $ 400    
Service [Member] | Minimum [Member]          
Disaggregation of Revenue [Line Items]          
Deferred service revenue, noncurrent, recognition period     2 years    
Service [Member] | Maximum [Member]          
Disaggregation of Revenue [Line Items]          
Deferred service revenue, noncurrent, recognition period     4 years