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Trade Accounts Receivable, Net (Tables)
 9 Months Ended
Sep. 30, 2018
Receivables [Abstract]  
Schedule of trade accounts receivable, net
Trade accounts receivable at the dates indicated consisted of the following: 
 
At September 30,
 
At December 31,
(in thousands)
2018
 
2017
 
2017
Trade accounts receivable
$
197,378

 
$
165,101

 
$
139,910

Allowance for doubtful accounts
(1,312
)
 
(1,166
)
 
(996
)
Allowance for sales discounts and returns
(3,085
)
 
(4,364
)
 
(2,956
)
 
$
192,981

 
$
159,571

 
$
135,958