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
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 647,326		$ 543,435		$ 1,019,043		$ 705,305		$ 536,520		$ 457,523		$ 733,546		$ 541,028		$ 2,915,109		$ 2,268,617		$ 1,880,905
Operating income (loss)	(19,295)		(37,666)		177,792		75,245		(32,051)		17,967		139,653		41,475		196,076		167,044		167,535
Net income (loss)	$ (16,255)		$ (14,274)		$ 140,266		$ 123,699		$ (36,523)		$ 18,957		$ 114,702		$ 34,929		$ 233,436		$ 132,065		$ 131,672
Earnings (loss) per share
Basic (in dollars per share)	$ (0.18)	[1]	$ (0.16)	[1]	$ 1.60	[1]	$ 1.42	[1]	$ (0.42)	[1]	$ 0.22	[1]	$ 1.33	[1]	$ 0.41	[1]	$ 2.66		$ 1.53		$ 1.53
Diluted (in dollars per share)	$ (0.18)	[1]	$ (0.16)	[1]	$ 1.59	[1]	$ 1.42	[1]	$ (0.42)	[1]	$ 0.22	[1]	$ 1.32	[1]	$ 0.40	[1]	$ 2.64	[2]	$ 1.52	[2]	$ 1.52	[2]

[1] The sum of quarterly amounts may not equal the annual amounts due to rounding.

[2] There were no anti-dilutive shares excluded from the calculation of diluted earnings per share for fiscal 2018, 2017 and 2016.