SUMMARIZED QUARTERLY FINANCIAL DATA (Unaudited) - Quarterly Operating Results (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenues	$ 271,796	$ 191,504	$ 256,824	$ 240,921	$ 114,520	$ 184,385	$ 105,765	$ 128,219	$ 961,045	$ 532,889	$ 878,554
Operating income (loss)	46,817	(17,822)	54,714	61,427	(68,596)	90,302	68,875	(301,811)	145,136	(211,230)	(1,437,851)
Net income (loss)	$ 262,430	$ (18,486)	$ 29,481	$ 33,403	$ (54,470)	$ 53,314	$ 36,759	$ (203,116)	$ 306,828	$ (167,513)	$ (945,731)
Diluted earnings per average common share (in dollars per share)	$ 2.68	$ (0.19)	$ 0.30	$ 0.34	$ (0.56)	$ 0.55	$ 0.38	$ (2.34)	$ 3.14	$ (1.77)	$ (12.43)
Basic earnings per average common share (in dollars per share)	$ 2.70	$ (0.19)	$ 0.30	$ 0.34	$ (0.56)	$ 0.55	$ 0.38	$ (2.34)	$ 3.16	$ (1.77)	$ (12.43)