Name:	ato_AccumulatedOtherComprehensiveIncomeLossDerivativesQualifyingasHedgesInterestRateAgreementsRealizedGainLossNetofTax
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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Anticipated debt issuance amount of future debt issuance.

No definition available.

Name:	ato_AnticipatedDebtIssuanceAmount
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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Gain or loss recognized in earnings during the reporting period representing the amount of the hedges' ineffectiveness for fair value and cash flow hedges.

No definition available.

Name:	ato_GainLossOnHedgeIneffectiveness
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Actual percentage of heating season gas purchases hedged using financial instruments.

No definition available.

Name:	ato_HedgePercentageOfActualHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Actual volume of heating season gas purchases hedged using financial instruments.

No definition available.

Name:	ato_HedgeVolumeOfActualHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	duration

Maximum anticipated percentage of heating season gas purchases hedged using financial instruments.

No definition available.

Name:	ato_MaximumHedgePercentageOfAnticipatedHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:percentItemType
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Minimum anticipated percentage of heating season gas purchases hedged using financial instruments.

No definition available.

Name:	ato_MinimumHedgePercentageOfAnticipatedHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:percentItemType
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Minimum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	ato_MinimumLengthOfTimeHedgedInCashFlowHedge
Namespace Prefix:	ato_
Data Type:	xbrli:durationItemType
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The domestic federal statutory tax rate applicable under enacted tax laws to the Nonregulated operations pretax income from continuing operations for the period. The "statutory" tax rate shall be the regular tax rate if there are alternative tax systems.

No definition available.

Name:	ato_NonregulatedSegmentEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
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Data Type:	num:percentItemType
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The domestic federal statutory tax rate applicable under enacted tax laws to the Regulated operations pretax income from continuing operations for the period. The "statutory" tax rate shall be the regular tax rate if there are alternative tax systems.

No definition available.

Name:	ato_RegulatedSegmentsEffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
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Data Type:	num:percentItemType
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Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
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Average stated rate on a group of interest rate forward or futures contracts.

No definition available.

Name:	us-gaap_DerivativeAverageForwardInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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Data Type:	xbrli:monetaryItemType
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_DerivativeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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- References

No definition available.

Name:	us-gaap_GeneralDiscussionOfDerivativeInstrumentsAndHedgingActivitiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994