Guarantor and Non-Guarantor financial information - Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	$ (21,200)	$ 90,872	$ 16,007	$ 448,023
Other comprehensive income (loss), net of income taxes:
Unrealized appreciation (depreciation) on securities, net	9,744	(3,307)	(2,719)	(46,754)
Minimum pension liability adjustments, net	314	197	940	590
Other comprehensive income (loss)	10,058	(3,110)	(1,779)	(46,164)
Comprehensive income (loss)	(11,142)	87,762	14,228	401,859
Reportable Legal Entities | Guarantor/Parent
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	(21,200)	90,872	16,007	448,023
Other comprehensive income (loss), net of income taxes:
Minimum pension liability adjustments, net	107	82	322	246
Other comprehensive income (loss)	107	82	322	246
Comprehensive income (loss)	(21,093)	90,954	16,329	448,269
Reportable Legal Entities | Issuer Subsidiary
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	(18,876)	91,451	21,160	450,937
Other comprehensive income (loss), net of income taxes:
Unrealized appreciation (depreciation) on securities, net	9,744	(3,307)	(2,719)	(46,754)
Minimum pension liability adjustments, net	207	115	618	344
Other comprehensive income (loss)	9,951	(3,192)	(2,101)	(46,410)
Comprehensive income (loss)	(8,925)	88,259	19,059	404,527
Reportable Legal Entities | Non-Guarantor Subsidiaries
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	(7,492)	14,001	(22,929)	25,671
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	(7,492)	14,001	(22,929)	25,671
Eliminations
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	26,368	(105,452)	1,769	(476,608)
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	$ 26,368	$ (105,452)	$ 1,769	$ (476,608)