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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Accounting Policies [Abstract]          
Allowance for doubtful accounts $ 2,779   $ 2,779   $ 1,949
Unearned revenue 17,692   17,692   7,353
Capitalized customer acquisition costs 10,100   10,100   $ 8,000
Amortization of customer acquisition costs     500    
Share-based compensation costs $ 2,600 $ 2,600 $ 8,900 $ 11,300