v3.8.0.1
ALLOWANCE FOR DOUBTFUL ACCOUNTS (Tables)
 12 Months Ended
Dec. 31, 2017
Accounts Receivable Additional Disclosures [Abstract]  
Allowance for Doubtful Accounts [Table Text Block]
The changes in the allowance for doubtful accounts consisted of the following:
 
Year Ended December 31,
 
2017
 
2016
 
2015
 
(Thousands of Dollars)
Balance as of beginning of year
$
7,756

 
$
8,473

 
$
7,808

Increase in allowance, net
2,217

 
24

 
965

Accounts charged against the allowance
(25
)
 
(741
)
 
(300
)
Balance as of end of year
$
9,948

 
$
7,756

 
$
8,473