Defined Benefit Pension and Other Postretirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 1,756	$ 1,751	$ 5,269	$ 5,426
Interest cost	5,594	5,564	16,782	16,483
Expected return on assets	(5,940)	(5,972)	(17,822)	(17,965)
Amortization of:
Prior service cost (benefit)	62	80	187	222
Actuarial loss	1,963	1,570	5,890	5,268
Net benefit cost	3,435	2,993	10,306	9,434
Change in associated regulatory liabilities	0	0	0	0
Net expense	3,435	2,993	10,306	9,434
Other Postretirement Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	43	54	129	161
Interest cost	165	182	497	547
Expected return on assets	(127)	(131)	(380)	(392)
Amortization of:
Prior service cost (benefit)	(105)	(174)	(317)	(522)
Actuarial loss	99	122	296	364
Net benefit cost	75	53	225	158
Change in associated regulatory liabilities	784	785	2,353	2,356
Net expense	$ 859	$ 838	$ 2,578	$ 2,514