GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Comprehensive Income (Loss) (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 COMPREHENSIVE INCOME (LOSS)
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)
Other comprehensive income (loss), net of income taxes:
Unrealized appreciation (depreciation) on securities, net									9,001	(829)	2,772
Reclassification of realized (gains) losses in net income, net											926
Minimum pension liability adjustments, net									5,249	3,333	(2,525)
Other comprehensive income									14,250	2,504	1,173
Comprehensive income (loss)									496,922	(125,708)	(55,655)
Eliminations
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)									(487,016)	124,224	61,638
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)									(487,016)	124,224	61,638
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)									482,672	(128,212)	(56,828)
Other comprehensive income (loss), net of income taxes:
Minimum pension liability adjustments, net									1,137	860	(63)
Other comprehensive income									1,137	860	(63)
Comprehensive income (loss)									483,809	(127,352)	(56,891)
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)									514,741	(113,892)	(46,856)
Other comprehensive income (loss), net of income taxes:
Unrealized appreciation (depreciation) on securities, net									9,001	(829)	2,772
Reclassification of realized (gains) losses in net income, net											926
Minimum pension liability adjustments, net									4,112	2,473	(2,462)
Other comprehensive income									13,113	1,644	1,236
Comprehensive income (loss)									527,854	(112,248)	(45,620)
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)									(27,725)	(10,332)	(14,782)
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)									$ (27,725)	$ (10,332)	$ (14,782)