<?xml version="1.0" encoding="utf-8"?>
<FilingSummary xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.4.0.6</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ContextCount>163</ContextCount>
  <ElementCount>291</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>52</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000100 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>true</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>000200 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/StatementCONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>000250 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (PARENTHETICALS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/StatementCONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITEDPARENTHETICALS</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (PARENTHETICALS)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>000300 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/StatementCONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMEUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>000350 - Statement - CONDENSED CONSOLIDATED STATEMENT OF INCOME (UNAUDITED) (PARENTHETICALS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/StatementCONDENSEDCONSOLIDATEDSTATEMENTOFINCOMEUNAUDITEDPARENTHETICALS</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENT OF INCOME (UNAUDITED) (PARENTHETICALS)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>000500 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/StatementCONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>001010 - Disclosure - 1. Nature of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure1NatureOfBusiness</Role>
      <ShortName>1. Nature of Business</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>001020 - Disclosure - 2. Unaudited Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure2UnauditedFinancialInformation</Role>
      <ShortName>2. Unaudited Financial Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>001030 - Disclosure - 3. Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure3FinancialInstruments</Role>
      <ShortName>3. Financial Instruments</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>001040 - Disclosure - 4. Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure4FairValueMeasurements</Role>
      <ShortName>4. Fair Value Measurements</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>001050 - Disclosure - 5. Discontinued Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure5DiscontinuedOperations</Role>
      <ShortName>5. Discontinued Operations</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>001060 - Disclosure - 6. Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure6Debt</Role>
      <ShortName>6. Debt</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>001070 - Disclosure - 7. Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure7EarningsPerShare</Role>
      <ShortName>7. Earnings Per Share</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>001080 - Disclosure - 8. Interim Pension and Other Postretirement Benefit Plan Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure8InterimPensionAndOtherPostretirementBenefitPlanInformation</Role>
      <ShortName>8. Interim Pension and Other Postretirement Benefit Plan Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>001090 - Disclosure - 9. Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure9CommitmentsAndContingencies</Role>
      <ShortName>9. Commitments and Contingencies</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>001100 - Disclosure - 10. Concentration of Credit Risk</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure10ConcentrationOfCreditRisk</Role>
      <ShortName>10. Concentration of Credit Risk</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>001110 - Disclosure - 11. Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/Disclosure11SegmentInformation</Role>
      <ShortName>11. Segment Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>002010 - Disclosure - Accounting Policies (Policy)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/AccountingPoliciesPolicy</Role>
      <ShortName>Accounting Policies (Policy)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>003020 - Disclosure - Unaudited Interim Financial Information (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureUnauditedInterimFinancialInformationTable</Role>
      <ShortName>Unaudited Interim Financial Information (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>003030 - Disclosure - Financial Instruments (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureFinancialInstrumentsTable</Role>
      <ShortName>Financial Instruments (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>003040 - Disclosure - Fair Value Measurements (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureFairValueMeasurementsTable</Role>
      <ShortName>Fair Value Measurements (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>003050 - Disclosure - Discontinued Operations (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureDiscontinuedOperationsTable</Role>
      <ShortName>Discontinued Operations (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>003060 - Disclosure - Debt (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureDebtTable</Role>
      <ShortName>Debt (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>003070 - Disclosure - Earnings Per Share (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureEarningsPerShareTable</Role>
      <ShortName>Earnings Per Share (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>003080 - Disclosure - Interim Pension and Other Postretirement Benefits (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureInterimPensionAndOtherPostretirementBenefitsTable</Role>
      <ShortName>Interim Pension and Other Postretirement Benefits (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>003090 - Disclosure - Commitments and Contingencies (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureCommitmentsAndContingenciesTable</Role>
      <ShortName>Commitments and Contingencies (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>003110 - Disclosure - Segment Information (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureSegmentInformationTable</Role>
      <ShortName>Segment Information (Table)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>004020 - Disclosure - Unaudited Interim Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureUnauditedInterimFinancialInformationDetails</Role>
      <ShortName>Unaudited Interim Financial Information (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>004030 - Disclosure - Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureFinancialInstrumentsDetails</Role>
      <ShortName>Financial Instruments (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>004040 - Disclosure - Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureFairValueMeasurementsDetails</Role>
      <ShortName>Fair Value Measurements (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>004045 - Disclosure - Fair Value Measurements Available-For-Sale Securities (Detials)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureFairValueMeasurementsAvailableForSaleSecuritiesDetials</Role>
      <ShortName>Fair Value Measurements Available-For-Sale Securities (Detials)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>004050 - Disclosure - Discontinued Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureDiscontinuedOperationsDetails</Role>
      <ShortName>Discontinued Operations (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>004060 - Disclosure - Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureDebtDetails</Role>
      <ShortName>Debt (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>004070 - Disclosure - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>004080 - Disclosure - Interim Pension and Other Postretirement Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureInterimPensionAndOtherPostretirementBenefitsDetails</Role>
      <ShortName>Interim Pension and Other Postretirement Benefits (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>004090 - Disclosure - Commitments and Contingencies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureCommitmentsAndContingenciesDetails</Role>
      <ShortName>Commitments and Contingencies (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>004110 - Disclosure - Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.atmosenergy.com/role/DisclosureSegmentInformationDetails</Role>
      <ShortName>Segment Information (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">Element ato_LineOfCreditFacilityCurrentAvailableBorrowingCapacity had a mix of decimals attribute values: -6 -5.</Log>
    <Log type="Info">Element us-gaap_DebtInstrumentInterestRateStatedPercentage had a mix of decimals attribute values: 4 5.</Log>
    <Log type="Info">'Monetary' elements on report '004030 - Disclosure - Financial Instruments (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '004040 - Disclosure - Fair Value Measurements (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '004060 - Disclosure - Debt (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Shares' elements on report '004070 - Disclosure - Earnings Per Share (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '004070 - Disclosure - Earnings Per Share (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '004080 - Disclosure - Interim Pension and Other Postretirement Benefits (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">'Monetary' elements on report '004090 - Disclosure - Commitments and Contingencies (Details)' had a mix of different decimal attribute values.</Log>
    <Log type="Info">Process Flow-Through: 000200 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED)</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Mar. 31, 2011'</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Sep. 30, 2010'</Log>
    <Log type="Info">Process Flow-Through: 000250 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED) (PARENTHETICALS)</Log>
    <Log type="Info">Process Flow-Through: 000300 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED)</Log>
    <Log type="Info">Process Flow-Through: 000350 - Statement - CONDENSED CONSOLIDATED STATEMENT OF INCOME (UNAUDITED) (PARENTHETICALS)</Log>
    <Log type="Info">Process Flow-Through: 000500 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</Log>
  </Logs>
  <InputFiles>
    <File>ato-20120331.xml</File>
    <File>ato-20120331.xsd</File>
    <File>ato-20120331_cal.xml</File>
    <File>ato-20120331_def.xml</File>
    <File>ato-20120331_lab.xml</File>
    <File>ato-20120331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
