QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Quarterly Financial Information [Line Items]
Revenues	$ 464,466	$ 238,280	$ 292,521	$ 638,464	$ 462,394	$ 247,142	$ 279,689	$ 550,408	$ 1,626,618
Operating income	80,855	36,241	41,043	130,290	93,825	45,093	48,365	129,445	288,429	$ 316,728	$ 288,947
Net income	$ 44,704	$ 16,276	$ 20,419	$ 90,835	$ 47,119	$ 18,797	$ 20,623	$ 76,456	$ 172,234	$ 162,995	$ 140,095
Basic	$ 0.85	$ 0.31	$ 0.39	$ 1.73	$ 0.90	$ 0.36	$ 0.39	$ 1.45	$ 3.27	$ 3.10	$ 2.67
Diluted	$ 0.84	$ 0.31	$ 0.39	$ 1.72	$ 0.89	$ 0.36	$ 0.39	$ 1.44	$ 3.25	$ 3.08	$ 2.65