<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>121</ContextCount>
  <ElementCount>79</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>17</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>75000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - EMPLOYEE BENEFIT PLAN, STATEMENTS OF NET ASSETS AVAILABLE FOR BENEFITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/StatementEmployeeBenefitPlanStatementsOfNetAssetsAvailableForBenefits</Role>
      <ShortName>EMPLOYEE BENEFIT PLAN, STATEMENTS OF NET ASSETS AVAILABLE FOR BENEFITS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75020 - Statement - EMPLOYEE BENEFIT PLAN, STATEMENT OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/StatementEmployeeBenefitPlanStatementOfChangesInNetAssetsAvailableForBenefits</Role>
      <ShortName>EMPLOYEE BENEFIT PLAN, STATEMENT OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75030 - Disclosure - DESCRIPTION OF PLAN</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureDescriptionOfPlan</Role>
      <ShortName>DESCRIPTION OF PLAN</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75040 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>75050 - Disclosure - RISKS AND UNCERTAINTIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureRisksAndUncertainties</Role>
      <ShortName>RISKS AND UNCERTAINTIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>75060 - Disclosure - PLAN TERMINATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosurePlanTermination</Role>
      <ShortName>PLAN TERMINATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>75070 - Disclosure - TAX STATUS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureTaxStatus</Role>
      <ShortName>TAX STATUS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>75080 - Disclosure - NEW YORK LIFE STABLE VALUE FUND</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureNewYorkLifeStableValueFund</Role>
      <ShortName>NEW YORK LIFE STABLE VALUE FUND</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>75090 - Disclosure - FAIR VALUE OF PLAN ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureFairValueOfPlanAssets</Role>
      <ShortName>FAIR VALUE OF PLAN ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>75100 - Disclosure - PARTY-IN-INTEREST TRANSACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosurePartyininterestTransactions</Role>
      <ShortName>PARTY-IN-INTEREST TRANSACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>75110 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureSubsequentEvents</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>75120 - Disclosure - SCHEDULE OF ASSETS (HELD AT END OF YEAR)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureScheduleOfAssetsHeldAtEndOfYear1</Role>
      <ShortName>SCHEDULE OF ASSETS (HELD AT END OF YEAR)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>75130 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>75140 - Disclosure - FAIR VALUE OF PLAN ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureFairValueOfPlanAssetsTables</Role>
      <ShortName>FAIR VALUE OF PLAN ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://unitil.com/20251231/taxonomy/role/DisclosureFairValueOfPlanAssets</ParentRole>
      <Position>15</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>75150 - Disclosure - DESCRIPTION OF PLAN (Additional Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureDescriptionOfPlanAdditionalInformationDetails</Role>
      <ShortName>DESCRIPTION OF PLAN (Additional Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://unitil.com/20251231/taxonomy/role/DisclosureDescriptionOfPlan</ParentRole>
      <Position>16</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>75160 - Disclosure - TAX STATUS (Additional Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureTaxStatusAdditionalInformationDetails</Role>
      <ShortName>TAX STATUS (Additional Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://unitil.com/20251231/taxonomy/role/DisclosureTaxStatus</ParentRole>
      <Position>17</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>75170 - Disclosure - FAIR VALUE OF PLAN ASSETS - Assets Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureFairValueOfPlanAssetsAssetsMeasuredAtFairValueOnARecurringBasisDetails</Role>
      <ShortName>FAIR VALUE OF PLAN ASSETS - Assets Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>75180 - Disclosure - PARTY-IN-INTEREST TRANSACTIONS (Additional Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosurePartyininterestTransactionsAdditionalInformationDetails</Role>
      <ShortName>PARTY-IN-INTEREST TRANSACTIONS (Additional Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://unitil.com/20251231/taxonomy/role/DisclosurePartyininterestTransactions</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>75190 - Disclosure - SUBSEQUENT EVENTS (Additional Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureSubsequentEventsAdditionalInformationDetails</Role>
      <ShortName>SUBSEQUENT EVENTS (Additional Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://unitil.com/20251231/taxonomy/role/DisclosureSubsequentEvents</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="utl-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>75200 - Disclosure - SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://unitil.com/20251231/taxonomy/role/DisclosureScheduleOfAssetsHeldAtEndOfYearDetails</Role>
      <ShortName>SCHEDULE OF ASSETS (HELD AT END OF YEAR) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://unitil.com/20251231/taxonomy/role/DisclosureScheduleOfAssetsHeldAtEndOfYear1</ParentRole>
      <Position>21</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="11-K" isDefinitelyFs="true" isUsgaap="true" original="utl-20251231.htm">utl-20251231.htm</File>
    <File>utl-20251231.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="247">http://fasb.org/us-gaap-ebp/2025</BaseTaxonomy>
    <BaseTaxonomy items="9">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
