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Valuation and Qualifying Accounts and Reserves (Schedule II)
 12 Months Ended
Dec. 31, 2016
Valuation and Qualifying Accounts [Abstract]  
Valuation and Qualifying Accounts and Reserves (Schedule II)
Valuation and Qualifying Accounts and Reserves
(Schedule II)
Changes in the allowance for doubtful accounts were as follows: 
(thousands of dollars)
2016
 
2015
 
2014
Beginning balance
$
49,420

 
$
53,770

 
$
54,460

Bad debt expense
29,869

 
30,393

 
34,810

Uncollectible accounts written off, net of recoveries
(38,839
)
 
(34,743
)
 
(35,500
)
Ending balance
$
40,450

 
$
49,420

 
$
53,770