SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 536,520	$ 457,523	$ 733,546	$ 541,028	$ 469,241	$ 393,213	$ 574,193	$ 444,258	$ 2,268,617		$ 1,880,905		$ 2,733,987
Operating income (loss)	(32,051)	17,967	139,653	41,475	42,480	(28,329)	93,933	59,451	167,044		167,535		248,451
Net income (loss)	$ (36,523)	$ 18,957	$ 114,702	$ 34,929	$ 25,400	$ (17,363)	$ 73,354	$ 50,281	$ 132,065		$ 131,672		$ 180,960
Earnings (loss) per share
Basic (in dollars per share)	$ (0.42)	$ 0.22	$ 1.33	$ 0.41	$ 0.30	$ (0.20)	$ 0.85	$ 0.59	$ 1.53		$ 1.53		$ 2.12
Diluted (in dollars per share)	$ (0.42)	$ 0.22	$ 1.32	$ 0.40	$ 0.29	$ (0.20)	$ 0.84	$ 0.58	$ 1.52	[1]	$ 1.52	[1]	$ 2.10	[1]

[1] There were no anti-dilutive shares excluded from the calculation of diluted earnings per share for fiscal 2017, 2016 and 2015.