SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)
Operating revenues	$ 696,825	$ 648,872	$ 577,484	$ 564,087	$ 532,304	$ 498,564	$ 405,283	$ 368,590	$ 2,487,268	$ 1,804,741	$ 1,624,232
Operating income (loss)	23,118	6,217	(1,253)	3,520	(29,677)	(28,028)	(65,672)	(49,164)	31,602	(172,541)	(25,966)
Income (loss) from continuing operations	2,175	(8,174)	(1,633)	500,642	(21,711)	(23,125)	(48,473)	(34,554)	493,010	(127,863)	(52,990)
Net income (loss)	$ 2,453	$ (8,008)	$ (11,879)	$ 500,106	$ (22,532)	$ (21,799)	$ (48,818)	$ (35,063)	$ 482,672	$ (128,212)	$ (56,828)
Basic earnings per common share:
Income (loss) from continuing operations (in dollars per share)	$ 0.02	$ (0.08)	$ (0.03)	$ 4.57	$ (0.20)	$ (0.22)	$ (0.45)	$ (0.33)	$ 4.49	$ (1.20)	$ (0.50)
Net income (loss) (in dollars per share)	0.02	(0.08)	(0.12)	4.57	(0.21)	(0.21)	(0.45)	(0.33)	4.39	(1.20)	(0.54)
Diluted earnings per common share:
Income (loss) from continuing operations (in dollars per share)	0.02	(0.08)	(0.03)	4.55	(0.20)	(0.22)	(0.45)	(0.33)	4.47	(1.20)	(0.50)
Net income (in dollars per share)	$ 0.02	$ (0.08)	$ (0.12)	$ 4.55	$ (0.21)	$ (0.21)	$ (0.45)	$ (0.33)	$ 4.37	$ (1.20)	$ (0.54)
Income tax expense (benefit) related to new tax act				$ 502,100					$ 502,100
Diluted earnings per share related to Tax Cuts and Jobs Act of 2017 (in dollars per share)				$ 4.59
Gain (loss) from the sale of assets, net	$ 5,500	$ 3,100	$ 3,800	$ 4,200	$ 2,300	$ 1,300	$ 10,100	$ 600
Gain (loss) from the sale of assets, per diluted share (in dollars per share)	$ 0.05	$ 0.02	$ 0.04	$ 0.04	$ 0.02	$ 0.01	$ 0.09	$ 0.01
Asset impairment, net of tax	$ 17,200
Asset impairment charge, net of tax per diluted share (in dollars per share)	$ 0.16