Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance			$ 987,905
Reclassifications, net of tax	$ 220	$ 239	660	$ 717
AOCI, net of tax - end balance	1,099,805	1,007,018	1,099,805	1,007,018
IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	592	501	1,776	1,477
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(8,555)	(11,356)	(8,995)	(11,834)
Reclassifications, net of tax	220	239	660	717
AOCI, net of tax - end balance	(8,335)	(11,117)	(8,335)	(11,117)
Postretirement Benefit Plans | IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	0	0	0	0
Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(16,612)	(18,808)	(17,796)	(19,784)
Reclassifications, net of tax	0	0	0	0
AOCI, net of tax - end balance	(16,020)	(18,307)	(16,020)	(18,307)
Derivative Instruments | IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	592	501	1,776	1,477
Total
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(25,167)	(30,164)	(26,791)	(31,618)
AOCI, net of tax - end balance	$ (24,355)	$ (29,424)	$ (24,355)	$ (29,424)