Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	$ 987,905
Reclassifications, net of tax	220	$ 239
AOCI, net of tax - end balance	1,042,021	959,736
IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	592	495
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(8,995)	(11,834)
Reclassifications, net of tax	220	239
AOCI, net of tax - end balance	(8,775)	(11,595)
Postretirement Benefit Plans | IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	0	0
Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(17,796)	(19,784)
Reclassifications, net of tax	0	0
AOCI, net of tax - end balance	(17,204)	(19,289)
Derivative Instruments | IRPAs
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	592	495
Total
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(26,791)	(31,618)
AOCI, net of tax - end balance	$ (25,979)	$ (30,884)