Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss)
Reclassifications, net of tax	$ 239	$ 160
IRPAs
Accumulated Other Comprehensive Income (Loss)
Net gains on IRPAs		1,877
Reclassifications, net of tax	495	390
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(11,834)	(9,276)
Reclassifications, net of tax	239	160
AOCI, net of tax - ending balance	(11,595)	(9,116)
Postretirement Benefit Plans | IRPAs
Accumulated Other Comprehensive Income (Loss)
Net gains on IRPAs		0
Reclassifications, net of tax	0	0
Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(19,784)	(4,410)
Reclassifications, net of tax	0	0
AOCI, net of tax - ending balance	(19,289)	(2,143)
Derivative Instruments | IRPAs
Accumulated Other Comprehensive Income (Loss)
Net gains on IRPAs		1,877
Reclassifications, net of tax	495	390
Total
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(31,618)	(13,686)
AOCI, net of tax - ending balance	$ (30,884)	$ (11,259)