SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 469,241	$ 393,213	$ 574,193	$ 444,258	$ 438,306	$ 458,467	$ 1,013,090	$ 824,124	$ 1,880,905		$ 2,733,987		$ 3,738,145
Operating income (loss)	42,480	(28,329)	93,933	59,451	6,257	(9,309)	82,806	168,697	167,535		248,451		201,190
Net income (loss)	$ 25,400	$ (17,363)	$ 73,354	$ 50,281	$ 4,197	$ (7,460)	$ 60,903	$ 123,320	$ 131,672		$ 180,960		$ 141,970
Earnings (loss) per share
Basic (in dollars per share)	$ 0.30	$ (0.20)	$ 0.85	$ 0.59	$ 0.05	$ (0.09)	$ 0.71	$ 1.46	$ 1.53		$ 2.12		$ 1.69
Diluted (in dollars per share)	$ 0.29	$ (0.20)	$ 0.84	$ 0.58	$ 0.05	$ (0.09)	$ 0.71	$ 1.44	$ 1.52	[1]	$ 2.10	[1]	$ 1.67	[1]

[1] There were no anti-dilutive shares excluded from the calculation of diluted earnings per share for fiscal 2016, 2015 and 2014.