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Revenue from Contracts with Customers - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Disaggregation of Revenue [Line Items]                    
Deferred revenue $ 10,023 $ 11,874 $ 10,023 $ 11,874 $ 10,023 $ 11,874 $ 10,980 $ 11,176 $ 11,971 $ 12,678
Amortization of capitalized commission costs 613 490 1,750 1,466            
Hardware and other revenue                    
Disaggregation of Revenue [Line Items]                    
Reserve for hardware returns         1.00% 2.00%        
Commissions and upfront payments made to a customer                    
Disaggregation of Revenue [Line Items]                    
Amortization of capitalized commission costs 600 $ 500 1,800 $ 1,500            
Activation Fees                    
Disaggregation of Revenue [Line Items]                    
Deferred revenue $ 8,400   $ 8,400   $ 8,400     $ 9,200