Name:	ato_CashFlowHedgeGainLossImpactonEarnings
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The amount of net gains or losses on a cash flow hedge reclassified to purchased gas cost from accumulated other comprehensive income or representing the ineffective portion of the hedge.

No definition available.

Name:	ato_CashFlowHedgeGainLossIncludedInCostOfSalesNet
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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The amount of net gains or losses on a cash flow hedge reclassified in the period to purchased gas cost from accumulated other comprehensive income

No definition available.

Name:	ato_CashFlowHedgeGainLossReclassifiedtoPurchasedGasCostNet
Namespace Prefix:	ato_
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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:	ato_CashFlowHedgeLineItems
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Data Type:	xbrli:stringItemType
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Gain (loss) recognized in earnings during the reporting period representing the amount of the fair value hedges' ineffectiveness due to a difference in basis.

No definition available.

Name:	ato_FairValueHedgeBasisIneffectiveness
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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Gain (loss) recognized in earnings during the reporting period representing the amount of the fair value hedges' ineffectiveness due to a difference in timing.

No definition available.

Name:	ato_FairValueHedgeTimingIneffectiveness
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of gain (loss) from the increase (decrease) in fair value of derivative and nonderivative instruments designated as fair value hedging instruments recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

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Amount of the increase (decrease) in fair value of the hedged item in a fair value hedge recognized in the income statement.

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Namespace Prefix:	us-gaap_
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- References

No definition available.

Name:	us-gaap_EffectOfFairValueHedgesOnResultsOfOperationsAbstract
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Data Type:	xbrli:stringItemType
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The net gain (loss) recognized in earnings during the reporting period representing the amount of the cash flow hedges' ineffectiveness.

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

Name:	us-gaap_GainLossOnCashFlowHedgeIneffectivenessNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Total amount of gain (loss) derived from fair value hedges recognized in earnings in the period.

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

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The amount of net gains or losses on interest rate cash flow hedges reclassified during the period to earnings from accumulated other comprehensive income upon the hedged transaction affecting earnings.

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