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Trade Accounts Receivable, net (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Receivables [Abstract]    
Trade accounts receivable $ 139,910 $ 116,368
Allowance for doubtful accounts (996) (895)
Allowance for sales discounts (2,956) (3,050)
Trade accounts receivable, net $ 135,958 $ 112,423