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Trade Accounts Receivable, Net (Tables)
 3 Months Ended
Mar. 31, 2018
Receivables [Abstract]  
Schedule of trade accounts receivable, net
Trade accounts receivable at the dates indicated consisted of the following: 
 
At March 31,
 
At December 31,
(in thousands)
2018
 
2017
 
2017
Trade accounts receivable
$
169,552

 
$
153,542

 
$
139,910

Allowance for doubtful accounts
(1,217
)
 
(1,284
)
 
(996
)
Allowance for sales discounts and returns
(1,189
)
 
(3,752
)
 
(2,956
)
 
$
167,146

 
$
148,506

 
$
135,958