Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Sep. 30, 2012 Bcf	Sep. 30, 2011	Sep. 30, 2010
Loss Contingency Information About Litigation Matters Abstract
Description of Pending Litigation	Since April 2009, Atmos Energy and two subsidiaries of AEH, Atmos Energy Marketing, LLC (AEM) and Atmos Gathering Company, LLC (AGC) (collectively, the Atmos Entities), have been involved in a lawsuit filed in the Circuit Court of Edmonson County, Kentucky related to our Park City Gathering Project. The dispute which gave rise to the litigation involves the amount of royalties due from a third party producer to landowners (who own the mineral rights) for natural gas produced from the landowners’ properties. The third party producer was operating pursuant to leases between the landowners and certain investors/working interest owners. The third party producer filed a petition in bankruptcy, which was subsequently dismissed due to the lack of meaningful assets to reorganize or liquidate. Although certain Atmos Energy companies entered into contracts with the third party producer to gather, treat and ultimately sell natural gas produced from the landowners’ properties, no Atmos Energy company had a contractual relationship with the landowners or the investors/working interest owners. After the lawsuit was filed, the landowners were successful in terminating for non-payment of royalties the leases related to the production of natural gas from their properties. Subsequent to termination, the investors/working interest owners under such leases filed additional claims against us for the termination of the leases. During the trial, the landowners and the investors/working interest owners requested an award of compensatory damages plus punitive damages against us. On December 17, 2010, the jury returned a verdict in favor of the landowners and investor/working interest owners and awarded compensatory damages of $3.8 million and punitive damages of $27.5 million payable by Atmos Energy and the two AEH subsidiaries. A hearing was held on February 28, 2011 to hear a number of motions, including a motion to dismiss the jury verdict and a motion for a new trial. The motions to dismiss the jury verdict and for a new trial were denied. However, the total punitive damages award was reduced from $27.5 million to $24.7 million. On October 17, 2011, we filed our brief of appellants with the Kentucky Court of Appeals (Court), appealing the verdict of the trial court. The appellees in this case subsequently filed their appellees’ brief with the Court on January 16, 2012, with our reply brief being filed with the Court on March 19, 2012. Oral arguments were held in the case on August 27, 2012; however, the Court has yet to render a decision. In addition, in a related development, on July 12, 2011, the Atmos Entities filed a lawsuit in the United States District Court, Western District of Kentucky, Atmos Energy Corporation et al.vs. Resource Energy Technologies, LLC and Robert Thorpe and John F. Charles, against the third party producer and its affiliates to recover all costs, including attorneys’ fees, incurred by the Atmos Entities, which are associated with the defense and appeal of the case discussed above as well as for all damages awarded to the plaintiffs in such case against the Atmos Entities. The total amount of damages being claimed in the lawsuit is “open-ended” since the appellate process and related costs are ongoing. This lawsuit is based upon the indemnification provisions agreed to by the third party producer in favor of Atmos Gathering that are contained in an agreement entered into between Atmos Gathering and the third party producer in May 2009. The defendants filed a motion to dismiss the case on August 25, 2011, with Atmos Energy filing a brief in response to such motion on September 19, 2011. On March 27, 2012 the court denied the motion to dismiss. Since that time, we have been engaged in discovery activities in this case. We have accrued what we believe is an adequate amount for the anticipated resolution of this matter; however, the amount accrued is less than the amount of the verdict. The Company does not have insurance coverage that could mitigate any losses that may arise from the resolution of this matter; however, we believe that the final outcome will not have a material adverse effect on our financial condition, results of operations or cash flows.
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required One To Three Years	29.0
Long Term Commitment Purchases [Abstract]
Significant Purchase Commitment Amount Description	$ 978,800,000	$ 1,498,600,000	$ 1,562,800,000
Long Term Purchase Commitment [Abstract]
Significant Purchase Commitment Remaining Minimum Amount Committed Next Fiscal Year	259,235,000
Significant Purchase Commitment Remaining Minimum Amount Committed Second Fiscal Year	74,604,000
Significant Purchase Commitment Remaining Minimum Amount Committed Third Fiscal Year	0
Significant Purchase Commitment Remaining Minimum Amount Committed Fourth Fiscal Year	0
Significant Purchase Commitment Remaining Minimum Amount Committed Fifth Fiscal Year	0
Significant Purchase Commitment Remaining Minimum Amount Committed Thereafter	0
Total Estimated Purchase Commitments	333,839,000
Estimated Contractual Demand Fees [Abstract]
Contractual Demand Fees, Demand Fees Due Next Fiscal Year	19,456,000
Contractual Demand Fees, Demand Fees Due Second Fiscal Year	10,554,000
Contractual Demand Fees, Demand Fees Due Third Fiscal Year	4,504,000
Contractual Demand Fees, Demand Fees Due Fourth Fiscal Year	278,000
Contractual Demand Fees, Demand Fees Due Fifth Fiscal Year	37,000
Contractual Demand Fees, Demand Fees Due Thereafter	128,000
Total Estimated Contractual Demand Fees	34,957,000
Loss Contingency, Settlement Agreement, Consideration	In December 2007, the Company received data requests from the Division of Investigations of the Office of Enforcement of the Federal Energy Regulatory Commission (the "Commission") in connection with its investigation into possible violations of the Commission’s posting and competitive bidding regulations for pre-arranged released firm capacity on natural gas pipelines. The Company and the Commission entered into a stipulation and consent agreement, which was approved by the Commission on December 9, 2011, thereby resolving this investigation. The Commission’s findings of violations were limited to the nonregulated operations of the Company. Under the terms of the agreement, the Company paid to the United States Treasury a total civil penalty of approximately $6.4 million and to energy assistance programs approximately $5.6 million in disgorgement of unjust profits plus interest for violations identified during the investigation. The resolution of this matter did not have a material adverse impact on the Company’s financial position, results of operations or cash flows and none of the payments were charged to any of the Company’s customers. In addition, none of the services the Company provides to any of its regulated or nonregulated customers were affected by the agreement.
Steel Service Line Replacement Cost	116,300,000
Inventories Under Indexed Contracts [Member]
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	72.2
Long Term Purchase Commitment Minimum Quantity Required After Three Years	29.0
Inventories Under Fixed Price Contracts [Member]
Long Term Purchase Commitment [Line Items]
Long Term Purchase Commitment Minimum Quantity Required Within One Year	3.8
Long Term Purchase Commitment Minimum Quantity Required One To Three Years	0.3
Purchase Commitment Amount Minimum	2.46
Purchase Commitment Amount Maximum	$ 6.36