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Revenue (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Disaggregation of Revenue [Line Items]          
Deferred service revenue, current $ 6,707   $ 6,707   $ 6,537
Deferred service revenue, non-current 1,454   1,454   890
Revenue 24,621 $ 21,578 41,046 $ 40,940  
Prepaid expenses and other current assets 2,538   2,538   $ 2,832
Service [Member]          
Disaggregation of Revenue [Line Items]          
Deferred service revenue 8,200   8,200    
Deferred service revenue, current 6,700   6,700    
Deferred service revenue, non-current 1,500   1,500    
Revenue 1,900   4,300    
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