Accumulated Other Comprehensive Income - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	$ 890,620	$ 839,837
Reclassifications, net of tax	639	517	$ 385
Net losses on derivative instruments	(16,942)	(4,105)	0
Benefit plans, principally actuarial losses	(3,197)	(3,482)	(1,413)
Balance, end of year	924,737	890,620	839,837
Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(9,276)	(6,311)	(5,283)
Reclassifications, net of tax	639	517	385
Net losses on derivative instruments	0	0
Benefit plans, principally actuarial losses	(3,197)	(3,482)	(1,413)
Balance, end of year	(11,834)	(9,276)	(6,311)
Derivative Instrument Net Losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(4,410)	(1,870)	(3,437)
Reclassifications, net of tax	0	0	0
Net losses on derivative instruments	(16,942)	(4,105)
Benefit plans, principally actuarial losses	0	0	0
Balance, end of year	(19,784)	(4,410)	(1,870)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(13,686)	(8,181)	(8,720)
Net losses on derivative instruments	(16,942)	(4,105)	0
Balance, end of year	(31,618)	(13,686)	(8,181)
Interest Rate Protection Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	1,568	1,565	1,567
Interest Rate Protection Agreements | Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	0	0	0
Interest Rate Protection Agreements | Derivative Instrument Net Losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	$ 1,568	$ 1,565	$ 1,567