SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 400,044	$ 298,974	$ 639,614	$ 615,036	$ 479,081	$ 434,942	$ 866,255	$ 811,767	$ 1,953,668	$ 2,592,045	$ 2,915,109
Operating income (loss)	39,862	(20,191)	95,215	101,497	(7,790)	(4,019)	77,001	88,743	216,383	153,935	199,882
Net income (loss)	$ 43,272	$ (27,219)	$ 88,505	$ 89,361	$ 18,086	$ (8,402)	$ 73,573	$ 86,248	$ 193,919	$ 169,505	$ 233,436
Earnings (loss) per share
Basic (in dollars per share)	$ 0.45	$ (0.28)	$ 0.93	$ 0.97	$ 0.20	$ (0.09)	$ 0.83	$ 0.97	$ 2.05	$ 1.90	$ 2.66
Diluted (in dollars per share)	$ 0.45	$ (0.28)	$ 0.92	$ 0.97	$ 0.20	$ (0.09)	$ 0.82	$ 0.97	$ 2.04	$ 1.89	$ 2.64