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Financial Instruments Commodity Contract Volume (Details)
Mar. 31, 2015
MMcf
Regulated Distribution Segment [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 7,533invest_InvestmentContractVolume
/ us-gaap_StatementBusinessSegmentsAxis
= ato_RegulatedDistributionSegmentMember
Regulated Distribution Segment [Member] | Nondesignated [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 7,533invest_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 133,767invest_InvestmentContractVolume
/ us-gaap_StatementBusinessSegmentsAxis
= ato_NonregulatedSegmentMember
Nonregulated Segment [Member] | Nondesignated [Member]  
Commodity Contract Outstanding Volumes [Line Items]  
Investment Contract Volume 86,114invest_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 (14,445)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 62,098invest_InvestmentContractVolume
/ us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis
= us-gaap_CashFlowHedgingMember
/ us-gaap_StatementBusinessSegmentsAxis
= ato_NonregulatedSegmentMember