Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Sep. 30, 2011
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 338,405	$ 300,455
Regulatory Liabilities	28,344	29,830
Postretirement benefits [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	13,147	11,476
Environmental overcollections [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	2,883	4,758
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	4,435	6,578
State tax benefits - distribution system repairs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	7,385	6,282
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	494	736
Income taxes recoverable [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	103,172	97,947
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	188,222	150,669
Environmental costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	16,812	19,547
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	11,602	12,163
Removal costs, net [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	12,718	12,313
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 5,879	$ 7,816