Name:	ato_CashCollateralRightToReclaimCash
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Data Type:	xbrli:monetaryItemType
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Aggregate net gain (loss) on all derivative instruments not designated as hedges recognized in revenue during the period, before tax effects.

No definition available.

Name:	ato_GainLossOnDerivativeInstrumentsNotDesignatedHedgesNetPretax
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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

The realized gain (loss) due to the settlement of interest rate agreements.

No definition available.

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

Actual percentage of heating season gas purchases 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 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 using financial instruments.

No definition available.

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

Minimum anticipated percentage of heating season gas purchases 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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Period Type:	duration

- Details

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Derivative instruments where an offsetting derivative position does not exist leading to an exposure to possible losses.

No definition available.

Name:	ato_NetOpenPositions
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Data Type:	xbrli:decimalItemType
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Period Type:	instant

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
Namespace Prefix:	ato_
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
Namespace Prefix:	ato_
Data Type:	num:percentItemType
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Amount of unrealized gain (loss), net of tax, related to the increase or decrease in fair value of interest rate derivatives designated as cash flow hedging instruments, which was recorded in accumulated other comprehensive income (loss) and being recognized as a component of interest expense over the life of the related financing arrangement.

No definition available.

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Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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Weighted average cost of heating season gas purchased using financial instruments.

No definition available.

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Namespace Prefix:	ato_
Data Type:	us-types:perUnitItemType
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Period Type:	duration

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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- Details

Name:	us-gaap_DerivativeLineItems
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Data Type:	xbrli:stringItemType
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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Charge to cost of goods sold that represents the reduction of the carrying amount of inventory, generally attributable to obsolescence or market conditions.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.2)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 330

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28360613&loc=d3e4542-108314

Name:	us-gaap_InventoryWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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=7668309&loc=d3e80748-113994

Name:	us-gaap_MaximumLengthOfTimeHedgedInCashFlowHedge1
Namespace Prefix:	us-gaap_
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Amount of unrealized gain (loss) related to the increase or decrease in fair value of interest rate derivatives designated as cash flow hedging instruments, which was recorded in accumulated other comprehensive income to the extent that the cash flow hedge was determined to be effective.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624171-113959