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ALLOWANCE FOR DOUBTFUL ACCOUNTS
 12 Months Ended
Dec. 31, 2017
ALLOWANCE FOR DOUBTFUL ACCOUNTS [Abstract]  
ALLOWANCE FOR DOUBTFUL ACCOUNTS
ALLOWANCE FOR DOUBTFUL ACCOUNTS

The following table shows the activity in the allowance for doubtful accounts (in thousands of dollars):
 
For the Years Ended December 31,
 
2017
 
2016
Balance at beginning of period
$
26,690

 
$
22,288

 Provision for uncollectible accounts
16,376

 
16,216

    Write-off of uncollectible accounts, net of recoveries
(16,597
)
 
(11,248
)
Foreign currency and other
2,798

 
(566
)
Balance at end of period
$
29,267

 
$
26,690