Unaudited Interim Financial Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2011
Regulatory Liabilities [Line Items]
Regulatory Asset - Replacement/renewal of distribution facilities	The amounts above do not include regulatory assets and liabilities related to our Missouri, Illinois and Iowa service areas, which are classified as assets held for sale as discussed in Note 5. During the prior fiscal year, the Railroad Commission of Texas’ Division of Public Safety issued a new rule requiring natural gas distribution companies to develop and implement a risk-based program for the renewal or replacement of distribution facilities, including steel service lines. The rule allows for the deferral of all expenses associated with capital expenditures incurred pursuant to this rule, including the recording of interest on the deferred expenses until the next rate proceeding (rate case or annual rate filing) at which time investment and costs would be recovered through base rates. As of December 31, 2011, we had deferred $0.1 million associated with the requirements of this rule which are recorded as other costs in the regulatory assets table above.
Statement Of Income And Comprehensive Income Abstract
Net income (loss)	$ 68,507	$ 73,997
Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax	901	776
Other Comprehensive Income Reclassification Adjustment And Unrealized Gain Loss On Derivatives Included In Net Income And Arising During Period Net Of Tax	(1,087)	31,847
Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax	(16,575)	10,350
Comprehensive Income Net Of Tax	51,746	116,970
Other Comprehensive Income Loss Tax Portion Attributable To Parent Abstract
Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax	514	455
Other Comprehensive Income Reclassification Adjustment And Unrealized Gain Loss On Derivatives Included In Net Income And Arising During Period Tax	(638)	18,704
Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Tax	(10,597)	6,617
Accumulated Other Comprehensive Income Loss Net Of Tax Abstract
Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax	3,459		2,558
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Interest Rate Hedges Effect Net Of Tax	(35,244)		(34,157)
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(33,436)		(16,861)
Accumulated other comprehensive income (loss)	(65,221)		(48,460)
Deferred Gas Costs Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	1,871		8,130
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	469,685		464,025
Miscellaneous Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	14,558		14,025
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	249,882		254,666
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory Assets	6,120		6,242
Deferred Gas Costs Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	88,799		33,976
Regulatory Cost of Removal Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	9,875		8,852
Environmental Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	288		385
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	4,493		4,862
Deferred Franchise Fees [Member]
Regulatory Asset [Line Items]
Regulatory Assets	365		379
Deferred Miscellaneous Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	$ 3,345		$ 3,534