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Financial Instruments Other Comprehensive Income (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Other Comprehensive Income (Loss), Net of Tax [Abstract]    
Interest rate agreements fair value $ (52,069)ato_OtherComprehensiveIncomeUnrealizedGainLossOnTreasuryLocksArisingDuringPeriodNetOfTax $ 13,270ato_OtherComprehensiveIncomeUnrealizedGainLossOnTreasuryLocksArisingDuringPeriodNetOfTax
Forward commodity contracts fair value (28,742)ato_OtherComprehensiveIncomeUnrealizedGainLossOnForwardCommodityContractsArisingDuringPeriodNetOfTax 6,226ato_OtherComprehensiveIncomeUnrealizedGainLossOnForwardCommodityContractsArisingDuringPeriodNetOfTax
Interest rate agreements 282ato_OtherComprehensiveIncomeReclassificationAdjustmentOnTreasuryLocksIncludedInNetIncomeNetOfTax 672ato_OtherComprehensiveIncomeReclassificationAdjustmentOnTreasuryLocksIncludedInNetIncomeNetOfTax
Forward commodity contracts (210)ato_OtherComprehensiveIncomeReclassificationAdjustmentOnForwardCommodityContractsIncludedInNetIncomeNetOfTax 1,592ato_OtherComprehensiveIncomeReclassificationAdjustmentOnForwardCommodityContractsIncludedInNetIncomeNetOfTax
Total other comprehensive income (loss) from hedging, net of tax (80,739)us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax [1] 21,760us-gaap_OtherComprehensiveIncomeLossDerivativesQualifyingAsHedgesNetOfTax [1]
Expected Earnings [Line Items]    
Interest Rate Agreement Gain (Loss) Reclassified To Earnings (18,910)ato_InterestRateAgreementGainLossReclassifiedToEarnings [2]  
Commodity Contract Gain (Loss) Reclassified To Earnings (30,626)ato_CommodityContractGainLossReclassifiedToEarnings [2]  
Total Hedge Gain (Loss) Reclassified To Earnings (49,536)ato_TotalHedgeGainLossReclassifiedToEarnings  
Next Twelve Months [Member]    
Expected Earnings [Line Items]    
Interest Rate Agreement Gain (Loss) Reclassified To Earnings (347)ato_InterestRateAgreementGainLossReclassifiedToEarnings
/ ato_ExpectedRecognitionInEarningsOfDeferredGainLossesFinancialInstrumentsAxis
= ato_NextTwelveMonthsMember
 
Commodity Contract Gain (Loss) Reclassified To Earnings (25,704)ato_CommodityContractGainLossReclassifiedToEarnings
/ ato_ExpectedRecognitionInEarningsOfDeferredGainLossesFinancialInstrumentsAxis
= ato_NextTwelveMonthsMember
 
Total Hedge Gain (Loss) Reclassified To Earnings (26,051)ato_TotalHedgeGainLossReclassifiedToEarnings
/ ato_ExpectedRecognitionInEarningsOfDeferredGainLossesFinancialInstrumentsAxis
= ato_NextTwelveMonthsMember
 
Thereafter [Member]    
Expected Earnings [Line Items]    
Interest Rate Agreement Gain (Loss) Reclassified To Earnings (18,563)ato_InterestRateAgreementGainLossReclassifiedToEarnings
/ ato_ExpectedRecognitionInEarningsOfDeferredGainLossesFinancialInstrumentsAxis
= ato_ThereafterMember
 
Commodity Contract Gain (Loss) Reclassified To Earnings (4,922)ato_CommodityContractGainLossReclassifiedToEarnings
/ ato_ExpectedRecognitionInEarningsOfDeferredGainLossesFinancialInstrumentsAxis
= ato_ThereafterMember
 
Total Hedge Gain (Loss) Reclassified To Earnings $ (23,485)ato_TotalHedgeGainLossReclassifiedToEarnings
/ ato_ExpectedRecognitionInEarningsOfDeferredGainLossesFinancialInstrumentsAxis
= ato_ThereafterMember
 
[1] Utilizing an income tax rate ranging from 37 percent to 39 percent based on the effective rates in each taxing jurisdiction.
[2] (1) Utilizing an income tax rate ranging from 37 percent to 39 percent based on the effective rates in each taxing jurisdiction.