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
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)
Operating revenues	$ 331,708	$ 366,486	$ 438,191	$ 487,847	$ 553,800	$ 661,445	$ 885,670	$ 1,060,787	$ 1,624,232	$ 3,161,702	$ 3,715,968
Operating income	(93,001)	(13,256)	41,621	38,670	(24,783)	135,049	231,326	330,371	(25,966)	671,963	1,053,174
Income (loss) from continuing operations	(72,869)	(21,193)	25,174	15,898	(27,590)	90,899	153,542	203,623	(52,990)	420,474	706,610
Net income (loss)	$ (72,835)	$ (21,200)	$ 21,205	$ 16,002	$ (27,596)	$ 90,872	$ 153,543	$ 203,608	$ (56,828)	$ 420,427	$ 706,563
Basic earnings per common share:
Income (loss) from continuing operations (in dollars per share)	$ (0.68)	$ (0.20)	$ 0.23	$ 0.15	$ (0.26)	$ 0.84	$ 1.42	$ 1.87	$ (0.50)	$ 3.88	$ 6.52
Net income (in dollars per share)	(0.68)	(0.20)	0.19	0.15	(0.26)	0.84	1.42	1.87	(0.54)	3.88	6.52
Diluted earnings per common share:
Income (loss) from continuing operations (in dollars per share)	(0.68)	(0.20)	0.23	0.15	(0.26)	0.83	1.41	1.86	(0.50)	3.85	6.44
Net income (in dollars per share)	$ (0.68)	$ (0.20)	$ 0.19	$ 0.15	$ (0.26)	$ 0.83	$ 1.41	$ 1.86	$ (0.54)	$ 3.85	$ 6.44
Increase in net loss due to change in accounting principle					$ 6,400
Increase in diluted loss per common share due to change in accounting principle					$ 0.06
Gain (loss) from the sale of assets, net	$ (1,400)		$ 1,500	$ 2,900	$ 1,900	$ 1,100	$ 1,800	$ 2,600
Gain (loss) from the sale of assets, per diluted share (in dollars per share)	$ (0.01)		$ 0.01	$ 0.03	$ 0.02	$ 0.01	$ 0.02	$ 0.02
Loss related to currency exchange losses, net of tax				$ 5,400
Loss related to currency exchange losses, per diluted share (in dollars per share)				$ 0.05
After-tax impairment charge		$ 2,900
After-tax impairment charge, per diluted share (in dollars per share)		$ 0.03
Loss from an other-than-temporary impairment of available-for-sale securities	$ 15,900
Loss from an other-than-temporary impairment of available-for-sale securities, per diluted share (in dollars per share)	$ 0.15
Loss from an litigation settlement, net of tax	$ 12,000
Loss from an litigation settlement, per diluted share (in dollars per share)	$ 0.11
After-tax abandonment charge							$ 6,700
After-tax abandonment charge, per diluted share (in dollars per share)							$ 0.06