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Summary of Significant Accounting Policies - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Accounting Policies [Abstract]          
Allowance for doubtful accounts $ 2,156   $ 2,156   $ 1,949
Unearned revenue 7,544   7,544   7,353
Capitalized customer acquisition costs 10,300   10,300   $ 8,000
Amortization of customer acquisition costs     300    
Share-based compensation costs $ 2,900 $ 2,900 $ 6,300 $ 8,700