Name:	ato_AccumulatedOtherComprehensiveIncomeLossDerivativesQualifyingasHedgesInterestRateAgreementsRealizedGainLossNetofTax
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Period Type:	instant

Anticipated debt issuance amount of future debt issuance.

No definition available.

Name:	ato_AnticipatedDebtIssuanceAmount
Namespace Prefix:	ato_
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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
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Period Type:	duration

Anticipated percentage of heating season gas purchases using financial instruments.

No definition available.

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

Anticipated volume of heating season gas purchases using financial instruments.

No definition available.

Name:	ato_HedgeVolumeOfAnticipatedHeatingSeasonGas
Namespace Prefix:	ato_
Data Type:	num:volumeItemType
Balance Type:	na
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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.

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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.

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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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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=64837297&loc=SL5624171-113959

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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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The amount of gain reclassified into earnings in the period when cash flow hedge is discontinued because it is probable that the original forecasted transactions will not occur by the end of the original period or an additional two month time period.

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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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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