Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012	Mar. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 322,329	$ 338,405	$ 295,689
Regulatory Liabilities	56,654	28,344	35,400
Postretirement benefits [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	13,865	13,147	12,053
Environmental overcollections [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	2,980	2,883	3,843
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	31,209	4,435	12,022
State tax benefits -- distribution system repairs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	7,900	7,385	6,735
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	700	494	747
Income taxes recoverable [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	104,229	103,172	99,226
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	181,305	188,222	146,631
Environmental costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	16,841	16,812	18,247
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	2,122	11,602	11,012
Removal costs, net [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	11,856	12,718	11,915
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 5,976	$ 5,879	$ 8,658