Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	$ (8,956)	$ (6,051)	$ (9,276)	$ (6,311)
Reclassifications, net of tax	160	128	480	388
AOCI, net of tax - ending balance	(8,796)	(5,923)	(8,796)	(5,923)
Postretirement Benefit Plans | Interest Rate Protection Agreements
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs			0
Reclassifications, net of tax	0	0	0	0
Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(20,607)	(1,087)	(4,410)	(1,870)
Reclassifications, net of tax	0	0	0	0
AOCI, net of tax - ending balance	(20,250)	(695)	(20,250)	(695)
Derivative Instruments | Interest Rate Protection Agreements
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs			(16,943)
Reclassifications, net of tax	357	392	1,103	1,175
AOCI Attributable to Parent
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(29,563)	(7,138)	(13,686)	(8,181)
Reclassifications, net of tax	160	128	480	388
AOCI, net of tax - ending balance	(29,046)	(6,618)	(29,046)	(6,618)
AOCI Attributable to Parent | Interest Rate Protection Agreements
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs			(16,943)
Reclassifications, net of tax	$ 357	$ 392	$ 1,103	$ 1,175