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Unaudited Interim Financial Information (Details) (USD $)
 3 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Regulatory Asset [Line Items]    
Regulatory assets $ 267,492,000us-gaap_RegulatoryAssets $ 250,309,000us-gaap_RegulatoryAssets
Regulatory Liabilities [Line Items]    
Regulatory liabilities 571,915,000us-gaap_RegulatoryLiabilities 545,759,000us-gaap_RegulatoryLiabilities
Deferred Gas Costs [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 61,530,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_DeferredGasCostsLiabilityMember		 35,063,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_DeferredGasCostsLiabilityMember
Deferred Franchise Fees [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 7,367,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_DeferredFranchiseFeesLiabilityMember		 5,268,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_DeferredFranchiseFeesLiabilityMember
Regulatory Cost of Removal Obligation [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 489,210,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_AssetRetirementObligationCostsMember		 490,448,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_AssetRetirementObligationCostsMember
Other [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 13,808,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_MiscellaneousRegulatoryLiabilitiesMember		 14,980,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_MiscellaneousRegulatoryLiabilitiesMember
Pension And Postretirement Benefit Costs [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 158,190,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_PensionAndPostretirementBenefitCostsMember		 [1] 162,777,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_PensionAndPostretirementBenefitCostsMember		 [1]
Regulatory asset - future recoverable pension costs 17,700,000ato_RegulatoryAssetFutureRecoverablePensionCosts
/ us-gaap_RegulatoryAssetAxis
= ato_PensionAndPostretirementBenefitCostsMember		 18,800,000ato_RegulatoryAssetFutureRecoverablePensionCosts
/ us-gaap_RegulatoryAssetAxis
= ato_PensionAndPostretirementBenefitCostsMember
Merger And Integration Costs Net [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 4,595,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_MergerAndIntegrationCostsNetMember		 4,730,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_MergerAndIntegrationCostsNetMember
Regulatory noncurrent asset, amortization period 20  
Deferred Gas Costs Asset [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 38,022,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_DeferredGasCostsAssetMember		 20,069,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_DeferredGasCostsAssetMember
Rate Case Costs [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 2,427,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_RateCaseCostsMember		 3,757,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_RateCaseCostsMember
Texas 8.209 [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 36,100,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_Texas8209Member		 [2] 26,948,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_Texas8209Member		 [2]
APT Annual Adjustment Mechanism [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 5,623,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_AptAnnualAdjustmentMechanismMember		 8,479,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_AptAnnualAdjustmentMechanismMember
Recoverable Loss On Reacquired Debt [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 18,238,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_RecoverableLossOnReacquiredDebtMember		 18,877,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_RecoverableLossOnReacquiredDebtMember
Other [Member]    
Regulatory Asset [Line Items]    
Regulatory assets $ 4,297,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_DeferredMiscellaneousCostsMember		 $ 4,672,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_DeferredMiscellaneousCostsMember
[1] Includes $17.7 million and $18.8 million of pension and postretirement expense deferred pursuant to regulatory authorization.
[2] Texas Rule 8.209 is a Railroad Commission rule that 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.