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Financial Instruments Commodity Contract Volume (Details)
Dec. 31, 2014
MMcf
Regulated Distribution Segment [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 16,493invest_InvestmentContractVolume
/ us-gaap_StatementBusinessSegmentsAxis
= ato_RegulatedDistributionSegmentMember
Regulated Distribution Segment [Member] | Nondesignated [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 16,493invest_InvestmentContractVolume
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_RegulatedDistributionSegmentMember
Regulated Distribution Segment [Member] | Fair Value Hedging [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 0invest_InvestmentContractVolume
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_RegulatedDistributionSegmentMember
Regulated Distribution Segment [Member] | Cash Flow Hedging [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 0invest_InvestmentContractVolume
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_RegulatedDistributionSegmentMember
Nonregulated Segment [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 125,245invest_InvestmentContractVolume
/ us-gaap_StatementBusinessSegmentsAxis
= ato_NonregulatedSegmentMember
Nonregulated Segment [Member] | Nondesignated [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 76,750invest_InvestmentContractVolume
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_NonregulatedSegmentMember
Nonregulated Segment [Member] | Fair Value Hedging [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume (17,225)invest_InvestmentContractVolume
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_FairValueHedgingMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_NonregulatedSegmentMember
Nonregulated Segment [Member] | Cash Flow Hedging [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 65,720invest_InvestmentContractVolume
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_NonregulatedSegmentMember