Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Sep. 30, 2011	Jun. 30, 2011
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 291,047	$ 300,455	$ 257,338
Regulatory Liabilities	33,978	29,830	46,357
Postretirement benefits [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	12,342	11,476	11,558
Environmental overcollections [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	3,726	4,758	6,182
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	10,324	6,578	22,451
State tax benefits -- distribution system repairs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	6,961	6,282	6,166
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	625	736	0
Income taxes recoverable [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	99,891	97,947	92,695
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	144,613	150,669	116,003
Environmental costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	16,562	19,547	20,712
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	9,829	12,163	7,836
Removal costs, net [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	11,840	12,313	11,240
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 8,312	$ 7,816	$ 8,852