Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 330,628	$ 338,405	$ 301,491
Regulatory Liabilities	26,799	28,344	28,409
Postretirement benefits [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	13,486	13,147	11,764
Environmental overcollections [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	3,077	2,883	4,740
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	1,916	4,435	5,019
State tax benefits -- distribution system repairs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	7,750	7,385	6,509
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	570	494	377
Income taxes recoverable [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	103,665	103,172	98,741
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	184,764	188,222	148,650
Environmental costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	17,119	16,812	19,372
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	7,797	11,602	14,798
Removal costs, net [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	11,541	12,718	11,946
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 5,742	$ 5,879	$ 7,984