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Significant Accounting Policies - Accounts and Receivable and Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]    
Balance at Beginning of Period $ 3,486 $ 1,090
Charged to Revenue 6,536 3,414
Charged to Costs and Expenses 1,331 824
Write-offs   (239)
Utilization (3,772) (1,603)
Balance at End of Period 7,581 3,486
Allowance for doubtful accounts    
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]    
Balance at Beginning of Period 1,175 590
Charged to Costs and Expenses 1,331 824
Write-offs   (239)
Balance at End of Period 2,506 1,175
Allowance for service changes and cancellations    
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]    
Balance at Beginning of Period 2,311 500
Charged to Revenue 6,536 3,414
Utilization (3,772) (1,603)
Balance at End of Period $ 5,075 $ 2,311