QUARTERLY FINANCIAL DATA (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2015

Schedule of Quarterly Financial Information [Line Items]

Quarterly Financial Data

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
Year Ended December 31, 2015
	(Thousands of dollars)
Revenues	$	676,531	$	256,786	$	225,226	$	389,149
Net margin	$	262,978	$	176,837	$	170,502	$	231,416
Operating income	$	109,005	$	31,270	$	24,951	$	73,903
Net income	$	60,381	$	12,076	$	7,371	$	39,202
Earnings per share
Basic	$	1.15	$	0.23	$	0.14	$	0.75
Diluted	$	1.13	$	0.23	$	0.14	$	0.74

	First Quarter		Second Quarter		Third Quarter		Fourth Quarter
Year Ended December 31, 2014
	(Thousands of dollars)
Revenues	$	766,178	$	296,838	$	241,522	$	514,368
Net margin	$	259,836	$	176,493	$	166,452	$	224,176
Operating income	$	109,353	$	26,812	$	19,119	$	70,010
Net income	$	59,076	$	9,454	$	4,653	$	36,607
Earnings per share
Basic	$	1.13	$	0.18	$	0.09	$	0.70
Diluted	$	1.13	$	0.18	$	0.09	$	0.69