Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance			$ 987,905
Reclassifications, net of tax	$ 220	$ 239	440	$ 478
AOCI, net of tax - end balance	1,117,016	1,010,581	1,117,016	1,010,581
IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	592	481	1,184	976
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(8,775)	(11,595)	(8,995)	(11,834)
Reclassifications, net of tax	220	239	440	478
AOCI, net of tax - end balance	(8,555)	(11,356)	(8,555)	(11,356)
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	(17,204)	(19,289)	(17,796)	(19,784)
Reclassifications, net of tax	0	0	0	0
AOCI, net of tax - end balance	(16,612)	(18,808)	(16,612)	(18,808)
Derivative Instruments | IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	592	481	1,184	976
Total
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(25,979)	(30,884)	(26,791)	(31,618)
AOCI, net of tax - end balance	$ (25,167)	$ (30,164)	$ (25,167)	$ (30,164)