Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance			$ 924,737
Reclassifications, net of tax	$ 239	$ 160	717	$ 480
AOCI, net of tax - end balance	1,007,018	938,899	1,007,018	938,899
IRPAs
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs				(16,943)
Reclassifications, net of tax	501	357	1,477	1,103
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(11,356)	(8,956)	(11,834)	(9,276)
Reclassifications, net of tax	239	160	717	480
AOCI, net of tax - end balance	(11,117)	(8,796)	(11,117)	(8,796)
Postretirement Benefit Plans | IRPAs
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	(18,808)	(20,607)	(19,784)	(4,410)
Reclassifications, net of tax	0	0	0	0
AOCI, net of tax - end balance	(18,307)	(20,250)	(18,307)	(20,250)
Derivative Instruments | IRPAs
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs				(16,943)
Reclassifications, net of tax	501	357	1,477	1,103
Total
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(30,164)	(29,563)	(31,618)	(13,686)
AOCI, net of tax - end balance	$ (29,424)	$ (29,046)	$ (29,424)	$ (29,046)