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TRADE RECEIVABLES (Tables)
 12 Months Ended
Dec. 31, 2017
TRADE RECEIVABLES [Abstract]  
Trade Receivables
Trade receivables, net, at December 31, 2017, and 2016, respectively, consisted of the following:
 
  
2017
  
2016
 
  
(thousands of dollars)
 
       
Trade receivables
 
$
26,079
  
$
22,493
 
Less allowance for doubtful accounts
  
(300
)
  
(300
)
         
  Trade receivables, net
 
$
25,779
  
$
22,193