Guarantor and Non-Guarantor financial information - Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 16,002	$ 203,608
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent
Unrealized depreciation on securities, net	(11,010)	(42,244)
Minimum pension liability adjustments, net	313	196
Other comprehensive loss	(10,697)	(42,048)
Comprehensive income	5,305	161,560
Reportable Legal Entities | Guarantor/Parent
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	16,002	203,608
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent
Minimum pension liability adjustments, net	107	82
Other comprehensive loss	107	82
Comprehensive income	16,109	203,690
Reportable Legal Entities | Issuer Subsidiary
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	17,300	204,606
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent
Unrealized depreciation on securities, net	(11,010)	(42,244)
Minimum pension liability adjustments, net	206	114
Other comprehensive loss	(10,804)	(42,130)
Comprehensive income	6,496	162,476
Reportable Legal Entities | Non-Guarantor Subsidiaries
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	(7,948)	7,471
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent
Comprehensive income	(7,948)	7,471
Eliminations
CONSOLIDATED CONDENSED STATEMENTS OF COMPREHENSIVE INCOME
Net income	(9,352)	(212,077)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent
Comprehensive income	$ (9,352)	$ (212,077)