Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	$ 924,737	$ 890,620
Reclassifications, net of tax	956	639	$ 517
Net losses on derivative instruments	0	(16,942)	(4,105)
Benefit plans, principally actuarial gains	1,883	(3,197)	(3,482)
Balance, end of year	987,905	924,737	890,620
Interest Rate Protection Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	1,988	1,568	1,565
Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(11,834)	(9,276)	(6,311)
Reclassifications, net of tax	956	639	517
Net losses on derivative instruments		0	0
Benefit plans, principally actuarial gains	1,883	(3,197)	(3,482)
Balance, end of year	(8,995)	(11,834)	(9,276)
Postretirement Benefit Plans | Interest Rate Protection Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	0	0	0
Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(19,784)	(4,410)	(1,870)
Reclassifications, net of tax	0	0	0
Net losses on derivative instruments		(16,942)	(4,105)
Benefit plans, principally actuarial gains	0	0	0
Balance, end of year	(17,796)	(19,784)	(4,410)
Derivative Instruments | Interest Rate Protection Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	1,988	1,568	1,565
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(31,618)	(13,686)	(8,181)
Net losses on derivative instruments	0	(16,942)	(4,105)
Balance, end of year	$ (26,791)	$ (31,618)	$ (13,686)