Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	$ 987,905	$ 924,737
Reclassifications, net of tax	872	956	$ 639
Net gains (losses) on derivative instruments	20	0	(16,942)
Benefit plans, principally actuarial gains	3,203	1,883	(3,197)
Balance, end of year	1,093,288	987,905	924,737
Interest Rate Protection Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	2,367	1,988	1,568
Postretirement Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(8,995)	(11,834)	(9,276)
Reclassifications, net of tax	872	956	639
Net gains (losses) on derivative instruments	0		0
Benefit plans, principally actuarial gains	3,203	1,883	(3,197)
Balance, end of year	(4,920)	(8,995)	(11,834)
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	(17,796)	(19,784)	(4,410)
Reclassifications, net of tax	0	0	0
Net gains (losses) on derivative instruments	20		(16,942)
Benefit plans, principally actuarial gains	0	0	0
Balance, end of year	(15,409)	(17,796)	(19,784)
Derivative Instruments | Interest Rate Protection Agreements
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassifications, net of tax	2,367	1,988	1,568
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(26,791)	(31,618)	(13,686)
Net gains (losses) on derivative instruments	20	0	(16,942)
Balance, end of year	$ (20,329)	$ (26,791)	$ (31,618)