Unaudited Interim Financial Information (Details) (USD $)	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2010
Regulatory Asset [Line Items]
Regulatory Assets	$ 271,615,000		$ 271,615,000		$ 277,510,000
Regulatory Liabilities [Line Items]
Regulatory Liabilities	457,888,000		457,888,000		430,919,000
Statement Of Income And Comprehensive Income Abstract
Net income (loss)	(566,000)	(3,154,000)	205,640,000	204,302,000
Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax	(94,000)	(1,696,000)	1,492,000	(337,000)
Other Comprehensive Income Reclassification Adjustment And Unrealized Gain Loss On Derivatives Included In Net Income And Arising During Period Net Of Tax	(7,884,000)	422,000	13,536,000	1,265,000
Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax	(285,000)	7,921,000	14,090,000	4,690,000
Comprehensive Income Net Of Tax	(8,829,000)	3,493,000	234,758,000	209,920,000
Other Comprehensive Income Loss Tax Portion Attributable To Parent Abstract
Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax	(56,000)	(996,000)	876,000	(198,000)
Other Comprehensive Income Reclassification Adjustment And Unrealized Gain Loss On Derivatives Included In Net Income And Arising During Period Tax	(4,629,000)	247,000	7,950,000	743,000
Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Tax	(182,000)	5,066,000	9,008,000	2,999,000
Accumulated Other Comprehensive Income Loss Net Of Tax Abstract
Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax	5,697,000		5,697,000		4,205,000
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Interest Rate Hedges Effect Net Of Tax	8,068,000		8,068,000		(5,468,000)
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(8,019,000)		(8,019,000)		(22,109,000)
Accumulated other comprehensive income (loss)	5,746,000		5,746,000		(23,372,000)
Other Information Abstract
One Time Tax Benefit			5,000,000
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	200,393,000		200,393,000		209,564,000
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory Assets	6,360,000		6,360,000		6,714,000
Deferred Gas Costs Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	22,083,000		22,083,000		22,701,000
Regulatory Cost of Removal Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	32,691,000		32,691,000		31,014,000
Environmental Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	434,000		434,000		805,000
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	5,321,000		5,321,000		4,505,000
Deferred Franchise Fees [Member]
Regulatory Asset [Line Items]
Regulatory Assets	393,000		393,000		1,161,000
Deferred Miscellaneous Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	3,940,000		3,940,000		1,046,000
Deferred Gas Costs Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	18,739,000		18,739,000		43,333,000
Deferred Franchise Fees Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	629,000		629,000		0
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	429,354,000		429,354,000		381,474,000
Miscellaneous Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 9,166,000		$ 9,166,000		$ 6,112,000