Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Sep. 30, 2012	Jun. 30, 2012
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 320,963	$ 338,405	$ 291,047
Regulatory Liabilities	40,002	28,344	33,978
Postretirement benefits [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	14,244	13,147	12,342
Environmental overcollections [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	2,906	2,883	3,726
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	14,156	4,435	10,325
State tax benefits -- distribution system repairs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	8,024	7,385	6,961
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	672	494	624
Income taxes recoverable [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	104,731	103,172	99,891
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	177,846	188,222	144,613
Environmental costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	16,642	16,812	16,562
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	4,109	11,602	9,829
Removal costs, net [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	12,074	12,718	11,840
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 5,561	$ 5,879	$ 8,312