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Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]    
Beginning balance, allowance for doubtful accounts $ 1,624 $ 679
Charges to expenses 462 1,733
Write-offs (335) (788)
Ending balance, allowance for doubtful accounts $ 1,751 $ 1,624