Guarantor and Non-Guarantor Financial Information - Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	$ 500,106	$ (35,063)
Other comprehensive income (loss), net of income taxes:
Unrealized (appreciation) depreciation on securities, net	(601)	12,412
Minimum pension liability adjustments, net	340	366
Other comprehensive income (loss)	(261)	12,778
Comprehensive income (loss)	499,845	(22,285)
Reportable Legal Entities | Guarantor/Parent
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	500,106	(35,063)
Other comprehensive income (loss), net of income taxes:
Minimum pension liability adjustments, net	102	106
Other comprehensive income (loss)	102	106
Comprehensive income (loss)	500,208	(34,957)
Reportable Legal Entities | Issuer Subsidiary
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	509,258	(32,865)
Other comprehensive income (loss), net of income taxes:
Unrealized (appreciation) depreciation on securities, net	(601)	12,412
Minimum pension liability adjustments, net	238	260
Other comprehensive income (loss)	(363)	12,672
Comprehensive income (loss)	508,895	(20,193)
Reportable Legal Entities | Non-Guarantor Subsidiaries
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	19,527	202
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	19,527	202
Eliminations
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income (loss)	(528,785)	32,663
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	$ (528,785)	$ 32,663