<?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>2.3.0.11</Version>
  <ProcessingTime />
  <ReportFormat>Html</ReportFormat>
  <ReportType />
  <ContextCount>163</ContextCount>
  <ElementCount>262</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>49</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report>
      <IsDefault>true</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000990 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document And Entity Information</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001000 - Statement - Condensed Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/CondensedConsolidatedStatementsOfOperationsUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Income (Unaudited)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>001010 - Statement - Condensed Consolidated Statements of Income (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/CondensedConsolidatedStatementsOfOperationsParentheticalUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Income (Unaudited) (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>002000 - Statement - Condensed Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows (Unaudited)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>003000 - Statement - Condensed Consolidated Balance Sheets (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/CondensedConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Unaudited)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>003010 - Statement - Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/CondensedConsolidatedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006010 - Disclosure - PROPOSED MERGER WITH AGL RESOURCES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ProposedMergerWithAglResources</Role>
      <ShortName>PROPOSED MERGER WITH AGL RESOURCES</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>006015 - Disclosure - Basis Of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/BasisOfPresentation</Role>
      <ShortName>Basis Of Presentation</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>006020 - Disclosure - ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/AccountingPolicies</Role>
      <ShortName>ACCOUNTING POLICIES</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>006030 - Disclosure - INVESTMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/Investments</Role>
      <ShortName>INVESTMENTS</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>006040 - Disclosure - SHORT-TERM AND LONG-TERM DEBT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ShortTermAndLongTermDebt</Role>
      <ShortName>SHORT-TERM AND LONG-TERM DEBT</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>006050 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/IncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>006060 - Disclosure - FAIR VALUE MEASUREMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/FairValueMeasurements</Role>
      <ShortName>FAIR VALUE MEASUREMENTS</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>006070 - Disclosure - DERIVATIVE INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/DerivativeInstruments</Role>
      <ShortName>DERIVATIVE INSTRUMENTS</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>006080 - Disclosure - POSTRETIREMENT BENEFITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/PostretirementBenefits</Role>
      <ShortName>POSTRETIREMENT BENEFITS</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>006090 - Disclosure - EQUITY INVESTMENT INCOME, NET</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/EquityInvestmentIncomeNet</Role>
      <ShortName>EQUITY INVESTMENT INCOME, NET</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>006100 - Disclosure - COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ComprehensiveIncome</Role>
      <ShortName>COMPREHENSIVE INCOME</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>006110 - Disclosure - BUSINESS SEGMENT INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/BusinessSegments</Role>
      <ShortName>BUSINESS SEGMENT INFORMATION</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>006120 - Disclosure - REGULATORY PROCEEDINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/RegulatoryProceedings</Role>
      <ShortName>REGULATORY PROCEEDINGS</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>006130 - Disclosure - GUARANTEES AND INDEMNITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/GuaranteesAndIndemnities</Role>
      <ShortName>GUARANTEES AND INDEMNITIES</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>006140 - Disclosure - CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/Contingencies</Role>
      <ShortName>CONTINGENCIES</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>007020 - Disclosure - ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/AccountingPoliciesPolicies</Role>
      <ShortName>ACCOUNTING POLICIES (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>007060 - Disclosure - FAIR VALUE MEASUREMENTS (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/FairValueMeasurementsPolicies</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>007070 - Disclosure - DERIVATIVE INSTRUMENTS (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/DerivativeInstrumentsPolicies</Role>
      <ShortName>DERIVATIVE INSTRUMENTS (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>007080 - Disclosure - POSTRETIREMENT BENEFITS (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/PostretirementBenefitsPolicies</Role>
      <ShortName>POSTRETIREMENT BENEFITS (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>007110 - Disclosure - BUSINESS SEGMENT INFORMATION (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/BusinessSegmentsPolicies</Role>
      <ShortName>BUSINESS SEGMENT INFORMATION (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>007130 - Disclosure - GUARANTEES AND INDEMNITIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/GuaranteesAndIndemnitiesPolicies</Role>
      <ShortName>GUARANTEES AND INDEMNITIES (Policies)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>008020 - Disclosure - ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/AccountingPoliciesTables</Role>
      <ShortName>ACCOUNTING POLICIES (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>008030 - Disclosure - INVESTMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/InvestmentsTables</Role>
      <ShortName>INVESTMENTS (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>008060 - Disclosure - FAIR VALUE MEASUREMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/FairValueMeasurementsTables</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>008070 - Disclosure - DERIVATIVE INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/DerivativeInstrumentsTables</Role>
      <ShortName>DERIVATIVE INSTRUMENTS (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>008080 - Disclosure - POSTRETIREMENT BENEFITS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/PostretirementBenefitsTables</Role>
      <ShortName>POSTRETIREMENT BENEFITS (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>008100 - Disclosure - COMPREHENSIVE INCOME (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ComprehensiveIncomeTables</Role>
      <ShortName>COMPREHENSIVE INCOME (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>008110 - Disclosure - BUSINESS SEGMENT INFORMATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/BusinessSegmentsTables</Role>
      <ShortName>BUSINESS SEGMENT INFORMATION (Tables)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>009010 - Disclosure - PROPOSED MERGER WITH AGL RESOURCES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/BasisOfPresentationDetails</Role>
      <ShortName>PROPOSED MERGER WITH AGL RESOURCES (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>009020 - Disclosure - ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/AccountingPoliciesDetails</Role>
      <ShortName>ACCOUNTING POLICIES (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>009030 - Disclosure - INVESTMENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/InvestmentsDetails</Role>
      <ShortName>INVESTMENTS (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>009040 - Disclosure - SHORT-TERM AND LONG-TERM DEBT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ShortTermAndLongTermDebtDetails</Role>
      <ShortName>SHORT-TERM AND LONG-TERM DEBT (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>009050 - Disclosure - INCOME TAXES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/IncomeTaxesDetails</Role>
      <ShortName>INCOME TAXES (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>009060 - Disclosure - FAIR VALUE MEASUREMENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/FairValueMeasurementsDetails</Role>
      <ShortName>FAIR VALUE MEASUREMENTS (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>009070 - Disclosure - DERIVATIVE INSTRUMENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/DerivativeInstrumentsDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>009080 - Disclosure - POSTRETIREMENT BENEFITS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/PostretirementBenefitsDetails</Role>
      <ShortName>POSTRETIREMENT BENEFITS (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>009090 - Disclosure - EQUITY INVESTMENT INCOME, NET (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/EquityInvestmentIncomeNetDetails</Role>
      <ShortName>EQUITY INVESTMENT INCOME, NET (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>009100 - Disclosure - COMPREHENSIVE INCOME (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ComprehensiveIncomeDetails</Role>
      <ShortName>COMPREHENSIVE INCOME (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>009110 - Disclosure - BUSINESS SEGMENT INFORMATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/BusinessSegmentsDetails</Role>
      <ShortName>BUSINESS SEGMENT INFORMATION (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>009120 - Disclosure - REGULATORY PROCEEDINGS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/RegulatoryProceedingsDetails</Role>
      <ShortName>REGULATORY PROCEEDINGS (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>009130 - Disclosure - GUARANTEES AND INDEMNITIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/GuaranteesAndIndemnitiesDetails</Role>
      <ShortName>GUARANTEES AND INDEMNITIES (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>009140 - Disclosure - CONTINGENCIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://nicor.com/role/ContingenciesDetails</Role>
      <ShortName>CONTINGENCIES (Details)</ShortName>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <ContainEmbeddedReports>false</ContainEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">Process Flow-Through: 001000 - Statement - Condensed Consolidated Statements of Income (Unaudited)</Log>
    <Log type="Info">Process Flow-Through: 001010 - Statement - Condensed Consolidated Statements of Income (Unaudited) (Parenthetical)</Log>
    <Log type="Info">Process Flow-Through: 002000 - Statement - Condensed Consolidated Statements of Cash Flows (Unaudited)</Log>
    <Log type="Info">Process Flow-Through: 003000 - Statement - Condensed Consolidated Balance Sheets (Unaudited)</Log>
    <Log type="Info">	Process Flow-Through: Removing column 'Dec. 31, 2009'</Log>
    <Log type="Info">Process Flow-Through: 003010 - Statement - Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical)</Log>
  </Logs>
  <InputFiles>
    <File>gas-20110630.xml</File>
    <File>gas-20110630.xsd</File>
    <File>gas-20110630_cal.xml</File>
    <File>gas-20110630_def.xml</File>
    <File>gas-20110630_lab.xml</File>
    <File>gas-20110630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles />
  <BaseTaxonomies />
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
