GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Comprehensive Income (Loss) (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 COMPREHENSIVE INCOME (LOSS)
Net income (loss)	$ 60,891	$ 18,959	$ (11,879)	$ 500,106	$ 79,850	$ 488,227
Other comprehensive income (loss), net of income taxes:
Unrealized appreciation (depreciation) on securities, net			(7,568)			(8,169)
Minimum pension liability adjustments, net	224		308		449	648
Other comprehensive income (loss)	224	$ 225	(7,260)	$ (261)	449	(7,521)
Comprehensive income (loss)	61,115		(19,139)		80,299	480,706
Eliminations
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	(62,079)		24,044		(99,780)	(504,741)
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	(62,079)		24,044		(99,780)	(504,741)
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	60,891		(11,880)		79,850	488,226
Other comprehensive income (loss), net of income taxes:
Minimum pension liability adjustments, net	77		92		155	194
Other comprehensive income (loss)	77		92		155	194
Comprehensive income (loss)	60,968		(11,788)		80,005	488,420
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	75,205		(8,000)		102,328	501,258
Other comprehensive income (loss), net of income taxes:
Unrealized appreciation (depreciation) on securities, net			(7,568)			(8,169)
Minimum pension liability adjustments, net	147		216		294	454
Other comprehensive income (loss)	147		(7,352)		294	(7,715)
Comprehensive income (loss)	75,352		(15,352)		102,622	493,543
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	(13,126)		(16,043)		(2,548)	3,484
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	$ (13,126)		$ (16,043)		$ (2,548)	$ 3,484