GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ 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
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue	$ 696,825	$ 648,872	$ 577,484	$ 564,087	$ 532,304	$ 498,564	$ 405,283	$ 368,590	$ 2,487,268	$ 1,804,741	$ 1,624,232
Operating costs and other									2,455,666	1,977,282	1,650,198
Operating income (loss) from continuing operations	23,118	6,217	(1,253)	3,520	(29,677)	(28,028)	(65,672)	(49,164)	31,602	(172,541)	(25,966)
Other (expense) income, net									8,504	7,690	(23,788)
Interest expense									(24,265)	(19,747)	(22,913)
Income (loss) from continuing operations before income taxes									15,841	(184,598)	(72,667)
Income tax (benefit) provision									(477,169)	(56,735)	(19,677)
Income (loss) from continuing operations	2,175	(8,174)	(1,633)	500,642	(21,711)	(23,125)	(48,473)	(34,554)	493,010	(127,863)	(52,990)
Income from discontinued operations before income taxes									23,389	3,285	2,360
Income tax provision									33,727	3,634	6,198
Loss from discontinued operations									(10,338)	(349)	(3,838)
Net income (loss)	$ 2,453	$ (8,008)	$ (11,879)	$ 500,106	$ (22,532)	$ (21,799)	$ (48,818)	$ (35,063)	482,672	(128,212)	(56,828)
Eliminations
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue									(87)	(67)	(70)
Operating costs and other									(938)	(882)	(1,323)
Operating income (loss) from continuing operations									851	815	1,253
Other (expense) income, net									(851)	(815)	(1,253)
Equity in net income (loss) of subsidiaries									(487,016)	124,224	61,638
Income (loss) from continuing operations before income taxes									(487,016)	124,224	61,638
Income (loss) from continuing operations									(487,016)	124,224	61,638
Net income (loss)									(487,016)	124,224	61,638
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating costs and other									14,276	16,566	13,145
Operating income (loss) from continuing operations									(14,276)	(16,566)	(13,145)
Other (expense) income, net									526	(240)	(194)
Interest expense									(499)	(398)	(375)
Equity in net income (loss) of subsidiaries									498,055	(116,212)	(47,166)
Income (loss) from continuing operations before income taxes									483,806	(133,416)	(60,880)
Income tax (benefit) provision									1,134	(5,204)	(4,052)
Income (loss) from continuing operations									482,672	(128,212)	(56,828)
Net income (loss)									482,672	(128,212)	(56,828)
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue									2,210,695	1,575,787	1,373,511
Operating costs and other									2,120,465	1,707,473	1,358,269
Operating income (loss) from continuing operations									90,230	(131,686)	15,242
Other (expense) income, net									7,363	7,342	(22,243)
Interest expense									(20,426)	(20,136)	(20,256)
Equity in net income (loss) of subsidiaries									(11,039)	(8,012)	(14,472)
Income (loss) from continuing operations before income taxes									66,128	(152,492)	(41,729)
Income tax (benefit) provision									(448,613)	(38,600)	5,127
Income (loss) from continuing operations									514,741	(113,892)	(46,856)
Net income (loss)									514,741	(113,892)	(46,856)
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue									276,660	229,021	250,791
Operating costs and other									321,863	254,125	280,107
Operating income (loss) from continuing operations									(45,203)	(25,104)	(29,316)
Other (expense) income, net									1,466	1,403	(98)
Interest expense									(3,340)	787	(2,282)
Income (loss) from continuing operations before income taxes									(47,077)	(22,914)	(31,696)
Income tax (benefit) provision									(29,690)	(12,931)	(20,752)
Income (loss) from continuing operations									(17,387)	(9,983)	(10,944)
Income from discontinued operations before income taxes									23,389	3,285	2,360
Income tax provision									33,727	3,634	6,198
Loss from discontinued operations									(10,338)	(349)	(3,838)
Net income (loss)									$ (27,725)	$ (10,332)	$ (14,782)