Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	$ (9,116)	$ (6,180)	$ (9,276)	$ (6,311)
Reclassifications, net of tax	160	129	320	260
AOCI, net of tax - ending balance	(8,956)	(6,051)	(8,956)	(6,051)
Postretirement Benefit Plans | Interest Rate Protection Agreements
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs	0		0
Reclassifications, net of tax	0	0	0	0
Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(2,143)	(1,479)	(4,410)	(1,870)
Reclassifications, net of tax	0	0	0	0
AOCI, net of tax - ending balance	(20,607)	(1,087)	(20,607)	(1,087)
Derivative Instruments | Interest Rate Protection Agreements
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs	(18,820)		(16,943)
Reclassifications, net of tax	356	392	746	783
AOCI Attributable to Parent
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(11,259)	(7,659)	(13,686)	(8,181)
Reclassifications, net of tax	160	129	320	260
AOCI, net of tax - ending balance	(29,563)	(7,138)	(29,563)	(7,138)
AOCI Attributable to Parent | Interest Rate Protection Agreements
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs	(18,820)		(16,943)
Reclassifications, net of tax	$ 356	$ 392	$ 746	$ 783