Name:	ato_CommodityContractGainLossReclassifiedToEarnings
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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_ExpectedEarningsLineItems
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The expected amount of gain or loss on interest rate agreement transaction(s) reclassified in the period to earnings from accumulated other comprehensive income.

No definition available.

Name:	ato_InterestRateAgreementGainLossReclassifiedToEarnings
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Net of tax effect of the reclassification adjustment for accumulated gains and losses from forward commodity contracts designated and qualifying as the effective portion of cash flow hedges included in accumulated comprehensive income that was realized in net income during the period.

No definition available.

Name:	ato_OtherComprehensiveIncomeReclassificationAdjustmentOnForwardCommodityContractsIncludedInNetIncomeNetOfTax
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Data Type:	xbrli:monetaryItemType
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Net of tax effect of the reclassification adjustment for accumulated gains and losses from interest rate agreements designated and qualifying as the effective portion of cash flow hedges included in accumulated comprehensive income that was realized in net income during the period.

No definition available.

Name:	ato_OtherComprehensiveIncomeReclassificationAdjustmentOnInterestRateAgreementsIncludedInNetIncomeNetOfTax
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Change in accumulated gains and losses from forward commodity contracts designated and qualifying as the effective portion of cash flow hedges, net of tax effect. The after tax effect change includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

No definition available.

Name:	ato_OtherComprehensiveIncomeUnrealizedGainLossOnForwardCommodityContractsArisingDuringPeriodNetOfTax
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Change in accumulated gains and losses from interest rate agreements designated and qualifying as the effective portion of cash flow hedges, net of tax effect. The after tax effect change includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

No definition available.

Name:	ato_OtherComprehensiveIncomeUnrealizedGainLossOnInterestRateAgreementsArisingDuringPeriodNetOfTax
Namespace Prefix:	ato_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

The expected amount of gains or losses on hedges reclassified in the period to earnings from accumulated other comprehensive income.

No definition available.

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

Amount after tax and reclassification adjustments, of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580

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- References

No definition available.