v3.19.3.a.u2
ALLOWANCE FOR DOUBTFUL ACCOUNTS
 12 Months Ended
Dec. 31, 2019
Accounts Receivable Additional Disclosures [Abstract]  
ALLOWANCE FOR DOUBTFUL ACCOUNTS ALLOWANCE FOR DOUBTFUL ACCOUNTS
The changes in the allowance for doubtful accounts consisted of the following:
 
Year Ended December 31,
 
2019
 
2018
 
2017
 
(Thousands of Dollars)
Balance as of beginning of year
$
9,412

 
$
9,380

 
$
7,700

Increase in allowance, net
2,322

 
233

 
1,705

Accounts charged against the allowance
(11,662
)
 
(201
)
 
(25
)
Balance as of end of year
$
72

 
$
9,412

 
$
9,380