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Summary of Significant Accounting Policies - Allowances for Doubtful Accounts and Sales Adjustments (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Balance at beginning of year $ 5,427 $ 3,396 $ 1,640
Provisions for losses - accounts receivable 2,286 4,110 4,484
Collection of accounts previously written off 0 0 0
Deductions for accounts charged off or credits issued (3,066) (2,079) (2,728)
Balance at end of year   5,427 $ 3,396
Accounts receivable, allowance for losses $ 4,647 $ 5,427