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
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)
Operating revenues	$ 532,304	$ 498,564	$ 405,283	$ 368,590	$ 331,708	$ 366,486	$ 438,191	$ 487,847	$ 1,804,741	$ 1,624,232	$ 3,161,702
Operating income (loss)	(29,677)	(28,028)	(65,672)	(49,164)	(93,001)	(13,256)	41,621	38,670	(172,541)	(25,966)	671,963
Income (loss) from continuing operations	(21,711)	(23,125)	(48,473)	(34,554)	(72,869)	(21,193)	25,174	15,898	(127,863)	(52,990)	420,474
Net income (loss)	$ (22,532)	$ (21,799)	$ (48,818)	$ (35,063)	$ (72,835)	$ (21,200)	$ 21,205	$ 16,002	$ (128,212)	$ (56,828)	$ 420,427
Basic earnings per common share:
Income (loss) from continuing operations (in dollars per share)	$ (0.20)	$ (0.22)	$ (0.45)	$ (0.33)	$ (0.68)	$ (0.20)	$ 0.23	$ 0.15	$ (1.20)	$ (0.50)	$ 3.88
Net income (in dollars per share)	(0.21)	(0.21)	(0.45)	(0.33)	(0.68)	(0.20)	0.19	0.15	(1.20)	(0.54)	3.88
Diluted earnings per common share:
Income (loss) from continuing operations (in dollars per share)	(0.20)	(0.22)	(0.45)	(0.33)	(0.68)	(0.20)	0.23	0.15	(1.20)	(0.50)	3.85
Net income (in dollars per share)	$ (0.21)	$ (0.21)	$ (0.45)	$ (0.33)	$ (0.68)	$ (0.20)	$ 0.19	$ 0.15	$ (1.20)	$ (0.54)	$ 3.85
Gain (loss) from the sale of assets, net	$ 2,300	$ 1,300	$ 10,100	$ 600	$ 1,400		$ 1,500	$ 2,900
Gain (loss) from the sale of assets, per diluted share (in dollars per share)	$ 0.02	$ 0.01	$ 0.09	$ 0.01	$ 0.01		$ 0.01	$ 0.03
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 after tax					$ 15,900
Loss from an other-than-temporary impairment of available-for-sale securities net of tax 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