Guarantor and Non-Guarantor financial information - Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 21,205	$ 153,543	$ 37,207	$ 357,151
Other comprehensive income (loss), net of income taxes:
Unrealized depreciation on securities, net	(1,453)	(1,203)	(12,463)	(43,447)
Minimum pension liability adjustments, net	313	197	626	393
Other comprehensive income (loss)	(1,140)	(1,006)	(11,837)	(43,054)
Comprehensive income (loss)	20,065	152,537	25,370	314,097
Reportable Legal Entities | Guarantor/Parent
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	21,205	153,543	37,207	357,151
Other comprehensive income (loss), net of income taxes:
Minimum pension liability adjustments, net	106	82	213	164
Other comprehensive income (loss)	106	82	213	164
Comprehensive income (loss)	21,311	153,625	37,420	357,315
Reportable Legal Entities | Issuer Subsidiary
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	22,736	154,880	40,036	359,486
Other comprehensive income (loss), net of income taxes:
Unrealized depreciation on securities, net	(1,453)	(1,203)	(12,463)	(43,447)
Minimum pension liability adjustments, net	207	115	413	229
Other comprehensive income (loss)	(1,246)	(1,088)	(12,050)	(43,218)
Comprehensive income (loss)	21,490	153,792	27,986	316,268
Reportable Legal Entities | Non-Guarantor Subsidiaries
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	(7,489)	4,199	(15,437)	11,670
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	(7,489)	4,199	(15,437)	11,670
Eliminations
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	(15,247)	(159,079)	(24,599)	(371,156)
Other comprehensive income (loss), net of income taxes:
Comprehensive income (loss)	$ (15,247)	$ (159,079)	$ (24,599)	$ (371,156)