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Summary of Significant Accounting Policies - Trade Receivables (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]      
Allowance for doubtful accounts, beginning balance $ 1,490 $ 1,517 $ 1,390
Charged to costs and expenses 1,407 1,601 1,322
Amounts charged off, net (948) (1,628) (1,195)
Allowance for doubtful accounts, ending balance $ 1,949 $ 1,490 $ 1,517