Pension and Other Post-Retirement Benefit Plans (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,033	$ 2,438	$ 9,098	$ 7,338
Interest cost	10,323	10,478	30,969	31,814
Expected return on plan assets	(11,136)	(11,419)	(33,409)	(34,593)
Amortization of prior service cost	268	262	805	793
Amortization of actuarial loss	3,670	1,050	11,009	3,207
Amortization of transition obligation	0	0	0	0
Partial settlement of the Supplemental Executive Retirement Program	0	3,529	0	3,529
Net periodic benefit cost	6,158	6,338	18,472	12,088
Amount allocated to construction projects	(698)	(265)	(2,211)	(789)
Amount deferred as regulatory asset/liability - net	(1,783)	(1,308)	(5,341)	(3,921)
Amount allocated to other	0	6	0	21
Amount charged to expense	3,677	4,771	10,920	7,399
Health and Life Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,856	1,648	5,568	4,944
Interest cost	6,278	6,331	18,833	18,993
Expected return on plan assets	(4,638)	(4,605)	(13,913)	(13,815)
Amortization of prior service cost	(980)	(1,005)	(2,941)	(3,016)
Amortization of actuarial loss	2,809	2,196	8,428	6,586
Amortization of transition obligation	272	272	815	815
Partial settlement of the Supplemental Executive Retirement Program	0	0	0	0
Net periodic benefit cost	5,597	4,837	16,790	14,507
Amount allocated to construction projects	(817)	(787)	(2,607)	(2,314)
Amount deferred as regulatory asset/liability - net	512	504	1,535	1,513
Amount allocated to other	0	0	0	0
Amount charged to expense	$ 5,292	$ 4,554	$ 15,718	$ 13,706