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Accounts Receivable - Summary of changes in allowance for doubtful accounts (Details 1) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]    
Beginning balance $ 4,471 $ 2,553
Bad debt expense 588 1,441
Accounts written-off (260) (105)
Recoveries 14 582
Ending balance $ 4,813 $ 4,471