GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2019	Mar. 31, 2018
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue	$ 720,868		$ 577,484		$ 1,461,466	$ 1,141,571
Operating costs and other	625,722		578,648		1,312,031	1,139,126
Operating income (loss) from continuing operations	95,146		(1,164)		149,435	2,445
Other (expense) income, net	7,956		1,637		(31,897)	3,802
Interest expense	(6,167)		(6,028)		(10,888)	(11,801)
Income (loss) from continuing operations before income taxes	96,935		(5,555)		106,650	(5,554)
Income tax (benefit) provision	25,078		(3,922)		26,429	(504,563)
Income (loss) from continuing operations	71,857		(1,633)		80,221	499,009
Income from discontinued operations before income taxes	2,889		1,263		15,554	744
Income tax provision	13,855		11,509		15,925	11,526
Loss from discontinued operations	(10,966)		(10,246)		(371)	(10,782)
Net income (loss)	60,891	$ 18,959	(11,879)	$ 500,106	79,850	488,227
Eliminations
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue	(24)		(21)		(48)	(37)
Operating costs and other	(270)		(246)		(539)	(451)
Operating income (loss) from continuing operations	246		225		491	414
Other (expense) income, net	(246)		(225)		(491)	(414)
Equity in net income (loss) of subsidiaries	(62,079)		24,044		(99,780)	(504,741)
Income (loss) from continuing operations before income taxes	(62,079)		24,044		(99,780)	(504,741)
Income (loss) from continuing operations	(62,079)		24,044		(99,780)	(504,741)
Net income (loss)	(62,079)		24,044		(99,780)	(504,741)
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating costs and other	2,818		4,120		5,584	8,209
Operating income (loss) from continuing operations	(2,818)		(4,120)		(5,584)	(8,209)
Other (expense) income, net	(45)		(70)		(42)	102
Interest expense	(6,080)		(138)		(7,119)	(166)
Equity in net income (loss) of subsidiaries	67,713		(7,963)		90,109	499,457
Income (loss) from continuing operations before income taxes	58,770		(12,291)		77,364	491,184
Income tax (benefit) provision	(2,121)		(411)		(2,486)	2,958
Income (loss) from continuing operations	60,891		(11,880)		79,850	488,226
Net income (loss)	60,891		(11,880)		79,850	488,226
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue	656,786		515,712		1,317,937	1,010,718
Operating costs and other	557,572		503,173		1,156,545	984,821
Operating income (loss) from continuing operations	99,214		12,539		161,392	25,897
Other (expense) income, net	7,875		1,732		(32,636)	3,373
Interest expense	(22)		(4,551)		(4,712)	(10,251)
Equity in net income (loss) of subsidiaries	(5,634)		(16,081)		9,671	5,284
Income (loss) from continuing operations before income taxes	101,433		(6,361)		133,715	24,303
Income tax (benefit) provision	26,228		1,639		31,387	(476,955)
Income (loss) from continuing operations	75,205		(8,000)		102,328	501,258
Net income (loss)	75,205		(8,000)		102,328	501,258
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF OPERATIONS
Operating revenue	64,106		61,793		143,577	130,890
Operating costs and other	65,602		71,601		150,441	146,547
Operating income (loss) from continuing operations	(1,496)		(9,808)		(6,864)	(15,657)
Other (expense) income, net	372		200		1,272	741
Interest expense	(65)		(1,339)		943	(1,384)
Income (loss) from continuing operations before income taxes	(1,189)		(10,947)		(4,649)	(16,300)
Income tax (benefit) provision	971		(5,150)		(2,472)	(30,566)
Income (loss) from continuing operations	(2,160)		(5,797)		(2,177)	14,266
Income from discontinued operations before income taxes	2,889		1,263		15,554	744
Income tax provision	13,855		11,509		15,925	11,526
Loss from discontinued operations	(10,966)		(10,246)		(371)	(10,782)
Net income (loss)	$ (13,126)		$ (16,043)		$ (2,548)	$ 3,484