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Trade accounts receivable - net (Tables)
 12 Months Ended
Dec. 31, 2017
Receivables [Abstract]  
Schedule of Accounts, Notes, Loans and Financing Receivable
The following table represents the activity occurring in the reserves for doubtful accounts in 2017, 2016 and 2015.
 
(in thousands)
 
Beginning Balance
at January 1
 
Net additions charged to
Operations
 
Deductions 
and Other
 
Ending Balance
at December 31
 
2017
 
$
365
 
$
136
 
$
221
 
$
722
 
2016
 
$
335
 
$
21
 
$
9
 
$
365
 
2015
 
$
244
 
$
99
 
$
(8)
 
$
335