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QUARTERLY FINANCIAL DATA (UNAUDITED) (Notes)
12 Months Ended
Dec. 31, 2017
Schedule of Quarterly Financial Information [Line Items]  
QUARTERLY FINANCIAL DATA (UNAUDITED)
14.
QUARTERLY FINANCIAL DATA (UNAUDITED)

 
 
First
Quarter
 
Second
Quarter
 
Third
Quarter
 
Fourth
Quarter
Year Ended December 31, 2017
 
 
 
 
 
 
(Thousands of dollars)
Revenues
 
$
550,408

 
$
279,689

 
$
247,142

 
$
462,394

Operating income
 
$
125,132

 
$
44,052

 
$
40,780

 
$
89,512

Net income
 
$
76,456

 
$
20,623

 
$
18,797

 
$
47,119

Earnings per share
 
 
 
 
 
 
 
 
   Basic
 
$
1.45

 
$
0.39

 
$
0.36

 
$
0.90

   Diluted
 
$
1.44

 
$
0.39

 
$
0.36

 
$
0.89

 
 
First
Quarter
 
Second
Quarter
 
Third
Quarter
 
Fourth
Quarter
Year Ended December 31, 2016
 
 
 
 
 
 
(Thousands of dollars)
Revenues
 
$
508,364

 
$
245,923

 
$
232,191

 
$
440,754

Operating income
 
$
116,073

 
$
43,621

 
$
30,892

 
$
78,534

Net income
 
$
64,743

 
$
20,300

 
$
12,737

 
$
42,315

Earnings per share
 
 
 
 
 
 
 
 
   Basic
 
$
1.23

 
$
0.39

 
$
0.24

 
$
0.81

   Diluted
 
$
1.22

 
$
0.38

 
$
0.24

 
$
0.80