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Unaudited Interim Financial Information (Details) (USD $)
 3 Months Ended
Mar. 31, 2015
Sep. 30, 2014
Regulatory Asset [Line Items]    
Regulatory assets $ 230,998,000us-gaap_RegulatoryAssets $ 250,309,000us-gaap_RegulatoryAssets
Regulatory Liabilities [Line Items]    
Regulatory liabilities 646,783,000us-gaap_RegulatoryLiabilities 545,759,000us-gaap_RegulatoryLiabilities
Deferred Gas Costs [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 135,069,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_DeferredFuelCostsMember		 35,063,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_DeferredFuelCostsMember
Deferred Franchise Fees [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 8,175,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_DeferredFranchiseFeesLiabilityMember		 5,268,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_DeferredFranchiseFeesLiabilityMember
Regulatory Cost of Removal [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 489,166,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember		 490,448,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_RemovalCostsMember
APT Annual Adjustment Mechanism [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 1,072,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_APTAnnualAdjustmentMechanismMember		 0us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= ato_APTAnnualAdjustmentMechanismMember
Other [Member]    
Regulatory Liabilities [Line Items]    
Regulatory liabilities 13,301,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember		 14,980,000us-gaap_RegulatoryLiabilities
/ us-gaap_RegulatoryLiabilityAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember
Pension And Postretirement Benefit Costs [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 153,381,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 [1] 162,777,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 [1]
Regulatory asset - future recoverable pension costs 16,400,000ato_RegulatoryAssetFutureRecoverablePensionCosts
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember		 18,800,000ato_RegulatoryAssetFutureRecoverablePensionCosts
/ us-gaap_RegulatoryAssetAxis
= us-gaap_PensionAndOtherPostretirementPlansCostsMember
Merger And Integration Costs Net [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 4,461,000us-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 [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 2,577,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember		 20,069,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_DeferredFuelCostsMember
Rate Case Costs [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 1,114,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_RateCaseCostsMember		 3,757,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_RateCaseCostsMember
Infrastructure Mechanisms [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 45,339,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_InfrastructureMechanismsMember		 [2] 26,948,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= ato_InfrastructureMechanismsMember		 [2]
APT Annual Adjustment Mechanism [Member]    
Regulatory Asset [Line Items]    
Regulatory assets 0us-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 17,598,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember		 18,877,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_LossOnReacquiredDebtMember
Other [Member]    
Regulatory Asset [Line Items]    
Regulatory assets $ 6,528,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember		 $ 4,672,000us-gaap_RegulatoryAssets
/ us-gaap_RegulatoryAssetAxis
= us-gaap_OtherRegulatoryAssetsLiabilitiesMember
[1] Includes $16.4 million and $18.8 million of pension and postretirement expense deferred pursuant to regulatory authorization.
[2] that allow for the deferral of all expenses associated with capital expenditures incurred pursuant to these rules, including the recording of interest until the next rate proceeding (rate case or annual rate filing), at which time investment and costs would be recovered through base rates.