Defined Benefit Pension and Other Postretirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 2,053	$ 1,756
Interest cost	5,196	5,594
Expected return on assets	(6,198)	(5,940)
Amortization of:
Prior service cost (benefit)	0	62
Actuarial loss	3,428	1,963
Net benefit cost	4,479	3,435
Change in associated regulatory liabilities	0	0
Net expense	4,479	3,435
Other Postretirement Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	51	43
Interest cost	143	166
Expected return on assets	(132)	(127)
Amortization of:
Prior service cost (benefit)	(105)	(106)
Actuarial loss	91	99
Net benefit cost	48	75
Change in associated regulatory liabilities	815	784
Net expense	$ 863	$ 859