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The expected recognition in earnings of deferred gains (losses) recorded in AOCI associated with our interest rate agreements.

No definition available.

Name:	ato_ExpectedRecognitioninEarningsofDeferredGainLossesInterestRateAgreements
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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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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
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Data Type:	xbrli:monetaryItemType
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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
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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
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Data Type:	xbrli:monetaryItemType
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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=66004124&loc=SL7669646-108580

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- References

No definition available.