GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Statement of Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	$ (72,835)	$ (21,200)	$ 21,205	$ 16,002	$ (27,596)	$ 90,872	$ 153,543	$ 203,608	$ (56,828)	$ 420,427	$ 706,563
Other comprehensive income, net of income taxes:
Unrealized (appreciation) depreciation on securities, net									2,772	(80,217)	(19,006)
Reclassification of realized (gains) losses in net income, net									926		(27,737)
Minimum pension liability adjustments, net									(2,525)	(4,286)	(2,661)
Other comprehensive income (loss)									1,173	(84,503)	(49,404)
Comprehensive income (loss)									(55,655)	335,924	657,159
Eliminations
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)									61,638	(414,214)	(715,525)
Other comprehensive income, net of income taxes:
Comprehensive income (loss)									61,638	(414,214)	(715,525)
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)									(56,828)	420,427	706,563
Other comprehensive income, net of income taxes:
Minimum pension liability adjustments, net									(63)	(666)	(213)
Other comprehensive income (loss)									(63)	(666)	(213)
Comprehensive income (loss)									(56,891)	419,761	706,350
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)									(46,856)	425,302	710,113
Other comprehensive income, net of income taxes:
Unrealized (appreciation) depreciation on securities, net									2,772	(80,217)	(19,006)
Reclassification of realized (gains) losses in net income, net									926		(27,737)
Minimum pension liability adjustments, net									(2,462)	(3,620)	(2,448)
Other comprehensive income (loss)									1,236	(83,837)	(49,191)
Comprehensive income (loss)									(45,620)	341,465	660,922
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)									(14,782)	(11,088)	5,412
Other comprehensive income, net of income taxes:
Comprehensive income (loss)									$ (14,782)	$ (11,088)	$ 5,412