Defined Benefit Pension and Other Postretirement Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	$ 2,052	$ 1,756	$ 6,158	$ 5,269
Interest cost	5,196	5,594	15,587	16,782
Expected return on assets	(6,197)	(5,940)	(18,594)	(17,822)
Amortization of:
Prior service cost (benefit)	62	62	187	187
Actuarial loss	3,366	1,963	10,098	5,890
Net benefit cost	4,479	3,435	13,436	10,306
Change in associated regulatory liabilities	0	0	0	0
Net expense	4,479	3,435	13,436	10,306
Other Postretirement Benefits [Member]
Components of net periodic pension expense and other postretirement benefit costs [Abstract]
Service cost	51	43	154	129
Interest cost	143	165	428	497
Expected return on assets	(133)	(127)	(396)	(380)
Amortization of:
Prior service cost (benefit)	(105)	(105)	(317)	(317)
Actuarial loss	91	99	273	296
Net benefit cost	47	75	142	225
Change in associated regulatory liabilities	815	784	2,446	2,353
Net expense	$ 862	$ 859	$ 2,588	$ 2,578