QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Quarterly Financial Information [Line Items]
Revenues	$ 514,368	$ 241,522	$ 296,838	$ 766,178	$ 522,686	$ 219,725	$ 311,608	$ 635,933	$ 1,818,906	$ 1,689,952	$ 1,376,649
Net margin	224,176	166,452	176,493	259,836	223,654	159,233	178,447	251,674	826,957	813,008	756,389
Operating income	70,010	19,119	26,812	109,353	65,014	14,189	39,307	101,838	225,294	220,348	215,714
Net income	$ 36,607	$ 4,653	$ 9,454	$ 59,076	$ 30,318	$ 434	$ 14,951	$ 53,492	$ 109,790	$ 99,195	$ 96,509
Basic	$ 0.70	$ 0.09	$ 0.18	$ 1.13					$ 2.10	$ 1.90	$ 1.84
Diluted	$ 0.69	$ 0.09	$ 0.18	$ 1.13					$ 2.07	$ 1.90	$ 1.84
Basic and Diluted					$ 0.58	$ 0.01	$ 0.29	$ 1.02