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Revenue (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Disaggregation of Revenue [Line Items]      
Deferred service revenue, current $ 6,428   $ 6,537
Deferred service revenue, non-current 769   890
Revenue 16,425 $ 19,362  
Prepaid expenses and other current assets 2,787   $ 2,832
Service [Member]      
Disaggregation of Revenue [Line Items]      
Deferred service revenue 7,200    
Deferred service revenue, current 6,400    
Deferred service revenue, non-current 800    
Revenue 2,400    
Prepaid expenses and other current assets $ 300    
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 3 years