Unaudited Interim Financial Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Sep. 30, 2011
Regulatory Asset [Line Items]
Regulatory Assets	$ 294,688		$ 294,688		$ 312,896
Regulatory Liabilities [Line Items]
Regulatory Liabilities	492,062		492,062		486,180
Regulatory Asset - Replacement/renewal of distribution facilities			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 March 31, 2012, we had deferred $0.7 million associated with the requirements of this rule which are recorded in “Other” in the regulatory assets table above.
Statement Of Income And Comprehensive Income Abstract
Net income (loss)	109,111	132,209	177,618	206,206
Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Net Of Tax	2,046	810	2,947	1,586
Other Comprehensive Income Reclassification Adjustment And Unrealized Gain Loss On Derivatives Included In Net Income And Arising During Period Net Of Tax	15,396	(10,427)	14,309	21,420
Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Net Of Tax	(5,315)	4,025	(21,890)	14,375
Comprehensive Income Net Of Tax	121,238	126,617	172,984	243,587
Other Comprehensive Income Loss Tax Portion Attributable To Parent Abstract
Other Comprehensive Income Unrealized Holding Gain Loss On Securities Arising During Period Tax	1,203	477	1,717	932
Other Comprehensive Income Reclassification Adjustment And Unrealized Gain Loss On Derivatives Included In Net Income And Arising During Period Tax	9,042	(6,125)	8,404	12,579
Other Comprehensive Income Unrealized Gain Loss On Derivatives Arising During Period Tax	(3,399)	2,573	(13,996)	9,190
Accumulated Other Comprehensive Income Loss Net Of Tax Abstract
Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax	5,505		5,505		2,558
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Interest Rate Hedges Effect Net Of Tax	(19,848)		(19,848)		(34,157)
Accumulated Other Comprehensive Income Loss Cumulative Changes In Net Gain Loss From Cash Flow Hedges Effect Net Of Tax	(38,751)		(38,751)		(16,861)
Accumulated other comprehensive income (loss)	(53,094)		(53,094)		(48,460)
Deferred Gas Costs Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	15,232		15,232		8,130
Asset Retirement Obligation Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	463,740		463,740		464,025
Miscellaneous Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory Liabilities	13,090		13,090		14,025
Pension And Postretirement Benefit Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	245,096		245,096		254,666
Merger And Integration Costs Net [Member]
Regulatory Asset [Line Items]
Regulatory Assets	5,998		5,998		6,242
Deferred Gas Costs Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	19,547		19,547		33,976
Regulatory Cost of Removal Asset [Member]
Regulatory Asset [Line Items]
Regulatory Assets	10,233		10,233		8,852
Environmental Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	117		117		385
Rate Case Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	4,503		4,503		4,862
Deferred Franchise Fees [Member]
Regulatory Asset [Line Items]
Regulatory Assets	333		333		379
Deferred Miscellaneous Costs [Member]
Regulatory Asset [Line Items]
Regulatory Assets	$ 8,861		$ 8,861		$ 3,534