Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance			$ 924,737
Reclassifications, net of tax	$ 239	$ 160	478	$ 320
AOCI, net of tax - end balance	1,010,581	940,199	1,010,581	940,199
IRPAs
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs		(18,820)		(16,943)
Reclassifications, net of tax	481	356	976	746
Postretirement Benefit Plans
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(11,595)	(9,116)	(11,834)	(9,276)
Reclassifications, net of tax	239	160	478	320
AOCI, net of tax - end balance	(11,356)	(8,956)	(11,356)	(8,956)
Postretirement Benefit Plans | IRPAs
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs		0		0
Reclassifications, net of tax	0	0	0	0
Derivative Instruments
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(19,289)	(2,143)	(19,784)	(4,410)
Reclassifications, net of tax	0	0	0	0
AOCI, net of tax - end balance	(18,808)	(20,607)	(18,808)	(20,607)
Derivative Instruments | IRPAs
Accumulated Other Comprehensive Income (Loss)
Net losses on IRPAs		(18,820)		(16,943)
Reclassifications, net of tax	481	356	976	746
Total
Accumulated Other Comprehensive Income (Loss)
AOCI, net of tax - beginning balance	(30,884)	(11,259)	(31,618)	(13,686)
AOCI, net of tax - end balance	$ (30,164)	$ (29,563)	$ (30,164)	$ (29,563)