Regulatory Assets and Liabilities and Regulatory Matters (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 233,997	$ 244,911	$ 330,628
Regulatory Liabilities	36,756	37,287	26,799
Postretirement benefits [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	16,846	16,497	13,486
Environmental overcollections [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	2,329	2,552	3,077
Deferred fuel and power refunds [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	7,524	8,283	1,916
State tax benefits -- distribution system repairs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	8,725	8,453	7,750
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Liabilities	1,332	1,502	570
Income taxes recoverable [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	106,435	106,069	103,665
Underfunded pension and postretirement plans [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	92,755	94,515	184,764
Environmental costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	14,910	17,054	17,119
Deferred fuel and power costs [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	427	8,283	7,797
Removal costs, net [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	13,748	13,333	11,541
Other [Member]
Regulatory Assets and Liabilities [Line Items]
Regulatory Assets	$ 5,722	$ 5,657	$ 5,742