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Financial Instruments Descriptions (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Mar. 31, 2015
MMcf
Mar. 31, 2014
General Discussion Of Derivative Instruments And Hedging Activities [Abstract]        
Minimum Hedge Percentage Of Anticipated Heating Season Gas     25.00%ato_MinimumHedgePercentageOfAnticipatedHeatingSeasonGas  
Maximum Hedge Percentage Of Anticipated Heating Season Gas     50.00%ato_MaximumHedgePercentageOfAnticipatedHeatingSeasonGas  
Hedge Percentage Of Actual Heating Season Gas     37.00%ato_HedgePercentageOfActualHeatingSeasonGas  
Hedge Volume Of Actual Heating Season Gas     28,200ato_HedgeVolumeOfActualHeatingSeasonGas  
Minimum Length Of Time Hedged In Cash Flow Hedge 1 month      
Maximum Length Of Time Hedged In Cash Flow Hedge 55 months      
Accumulated Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Interest Rate Agreements, Realized Gain (Loss), Net of Tax $ (18.8)ato_AccumulatedOtherComprehensiveIncomeLossDerivativesQualifyingasHedgesInterestRateAgreementsRealizedGainLossNetofTax   $ (18.8)ato_AccumulatedOtherComprehensiveIncomeLossDerivativesQualifyingasHedgesInterestRateAgreementsRealizedGainLossNetofTax  
Gain (Loss) On Hedge Ineffectiveness (2.3)ato_GainLossOnHedgeIneffectiveness (3.7)ato_GainLossOnHedgeIneffectiveness (4.5)ato_GainLossOnHedgeIneffectiveness 1.4ato_GainLossOnHedgeIneffectiveness
Regulated Segments Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate     37.00%ato_RegulatedSegmentsEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Nonregulated Segment Effective Income Tax Rate Reconciliation At Federal Statutory Income Tax Rate     39.00%ato_NonregulatedSegmentEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate  
Gain (Loss) On Derivative Instruments Not Designated Hedges Net Pretax 8.7us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet (9.3)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet 9.6us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet (10.1)us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
Interest Rate Hedges, Senior Notes 2017 Issuance [Member]        
Derivative [Line Items]        
Notional Amount of Interest Rate Cash Flow Hedge Derivatives 250invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2017IssuanceMember
  250invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2017IssuanceMember
 
Anticipated Debt Issuance, Amount 250ato_AnticipatedDebtIssuanceAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2017IssuanceMember
  250ato_AnticipatedDebtIssuanceAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2017IssuanceMember
 
Derivative, Average Forward Interest Rate 3.37%us-gaap_DerivativeAverageForwardInterestRate
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2017IssuanceMember
  3.37%us-gaap_DerivativeAverageForwardInterestRate
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2017IssuanceMember
 
Interest Rate Hedges, Senior Notes 2019 Issuance [Member]        
Derivative [Line Items]        
Notional Amount of Interest Rate Cash Flow Hedge Derivatives 450invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2019IssuanceMember
  450invest_DerivativeNotionalAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2019IssuanceMember
 
Anticipated Debt Issuance, Amount $ 450ato_AnticipatedDebtIssuanceAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2019IssuanceMember
  $ 450ato_AnticipatedDebtIssuanceAmount
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2019IssuanceMember
 
Derivative, Average Forward Interest Rate 3.78%us-gaap_DerivativeAverageForwardInterestRate
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2019IssuanceMember
  3.78%us-gaap_DerivativeAverageForwardInterestRate
/ us-gaap_DerivativeByNatureAxis
= ato_InterestRateHedgesSeniorNotes2019IssuanceMember