<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.3</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>190</ContextCount>
  <ElementCount>268</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>58</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>9952151 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMEUnaudited</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF INCOME (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952152 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUnaudited</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952153 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUnauditedParentheticals</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUnaudited</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUnauditedParentheticals</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952156 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CAPITAL STOCK AND LONG-TERM DEBT (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCAPITALSTOCKANDLONGTERMDEBTUnaudited</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CAPITAL STOCK AND LONG-TERM DEBT (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952157 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CAPITAL STOCK AND LONG-TERM DEBT (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCAPITALSTOCKANDLONGTERMDEBTUnauditedParentheticals</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CAPITAL STOCK AND LONG-TERM DEBT (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952158 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMMONSTOCKHOLDERSEQUITYUnaudited</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952159 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMMONSTOCKHOLDERSEQUITYUnauditedParentheticals</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF COMMON STOCKHOLDERS' EQUITY (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Basis of Presentation and Recent Developments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BasisofPresentationandRecentDevelopments</Role>
      <ShortName>Basis of Presentation and Recent Developments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Rate and Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/RateandRegulatoryMatters</Role>
      <ShortName>Rate and Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Capitalization</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/Capitalization</Role>
      <ShortName>Capitalization</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Business Segment Data</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BusinessSegmentData</Role>
      <ShortName>Business Segment Data</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Short-term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/ShorttermBorrowings</Role>
      <ShortName>Short-term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Commitments and Contingent Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CommitmentsandContingentLiabilities</Role>
      <ShortName>Commitments and Contingent Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Employee Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EmployeeBenefitPlans</Role>
      <ShortName>Employee Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Revenue Recognition from Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/RevenueRecognitionfromContractswithCustomers</Role>
      <ShortName>Revenue Recognition from Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Basis of Presentation and Recent Developments (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BasisofPresentationandRecentDevelopmentsPolicies</Role>
      <ShortName>Basis of Presentation and Recent Developments (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Basis of Presentation and Recent Developments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BasisofPresentationandRecentDevelopmentsTables</Role>
      <ShortName>Basis of Presentation and Recent Developments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/BasisofPresentationandRecentDevelopments</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Capitalization (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CapitalizationTables</Role>
      <ShortName>Capitalization (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/Capitalization</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/EarningsPerShare</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Business Segment Data (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BusinessSegmentDataTables</Role>
      <ShortName>Business Segment Data (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/BusinessSegmentData</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Short-term Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/ShorttermBorrowingsTables</Role>
      <ShortName>Short-term Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/ShorttermBorrowings</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Commitments and Contingent Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CommitmentsandContingentLiabilitiesTables</Role>
      <ShortName>Commitments and Contingent Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/CommitmentsandContingentLiabilities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Employee Benefit Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EmployeeBenefitPlansTables</Role>
      <ShortName>Employee Benefit Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/EmployeeBenefitPlans</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Revenue Recognition from Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/RevenueRecognitionfromContractswithCustomersTables</Role>
      <ShortName>Revenue Recognition from Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/RevenueRecognitionfromContractswithCustomers</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Basis of Presentation and Recent Developments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BasisofPresentationandRecentDevelopmentsDetails</Role>
      <ShortName>Basis of Presentation and Recent Developments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/BasisofPresentationandRecentDevelopmentsTables</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Rate and Regulatory Matters (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/RateandRegulatoryMattersDetails</Role>
      <ShortName>Rate and Regulatory Matters (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/RateandRegulatoryMatters</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Capitalization - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CapitalizationNarrativeDetails</Role>
      <ShortName>Capitalization - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Capitalization - Schedule of Carrying Amount and Fair Value of First Mortgage Bonds (FMBs) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CapitalizationScheduleofCarryingAmountandFairValueofFirstMortgageBondsFMBsDetails</Role>
      <ShortName>Capitalization - Schedule of Carrying Amount and Fair Value of First Mortgage Bonds (FMBs) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/EarningsPerShareTables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Business Segment Data - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BusinessSegmentDataNarrativeDetails</Role>
      <ShortName>Business Segment Data - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Business Segment Data - Schedule of Segment Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/BusinessSegmentDataScheduleofSegmentTransactionsDetails</Role>
      <ShortName>Business Segment Data - Schedule of Segment Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Short-term Borrowings - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/ShorttermBorrowingsNarrativeDetails</Role>
      <ShortName>Short-term Borrowings - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Short-term Borrowings - Schedule of Lines of Credit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/ShorttermBorrowingsScheduleofLinesofCreditDetails</Role>
      <ShortName>Short-term Borrowings - Schedule of Lines of Credit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Short-term Borrowings - Schedule of Weighted Average Daily Amounts of Borrowings Outstanding and Weighted Average Interest Rates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/ShorttermBorrowingsScheduleofWeightedAverageDailyAmountsofBorrowingsOutstandingandWeightedAverageInterestRatesDetails</Role>
      <ShortName>Short-term Borrowings - Schedule of Weighted Average Daily Amounts of Borrowings Outstanding and Weighted Average Interest Rates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Commitments and Contingent Liabilities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CommitmentsandContingentLiabilitiesNarrativeDetails</Role>
      <ShortName>Commitments and Contingent Liabilities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Commitments and Contingent Liabilities - Schedule of Purchased Water Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CommitmentsandContingentLiabilitiesScheduleofPurchasedWaterCostsDetails</Role>
      <ShortName>Commitments and Contingent Liabilities - Schedule of Purchased Water Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Commitments and Contingent Liabilities - Schedule of Operating Lease ROU Assets and Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CommitmentsandContingentLiabilitiesScheduleofOperatingLeaseROUAssetsandLeaseLiabilitiesDetails</Role>
      <ShortName>Commitments and Contingent Liabilities - Schedule of Operating Lease ROU Assets and Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Commitments and Contingent Liabilities - Schedule of Future Minimum Operating Lease Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/CommitmentsandContingentLiabilitiesScheduleofFutureMinimumOperatingLeaseCommitmentsDetails</Role>
      <ShortName>Commitments and Contingent Liabilities - Schedule of Future Minimum Operating Lease Commitments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Employee Benefit Plans - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EmployeeBenefitPlansNarrativeDetails</Role>
      <ShortName>Employee Benefit Plans - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Employee Benefit Plans - Schedule of Employee Retirement Benefit Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/EmployeeBenefitPlansScheduleofEmployeeRetirementBenefitPlansDetails</Role>
      <ShortName>Employee Benefit Plans - Schedule of Employee Retirement Benefit Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Revenue Recognition from Contracts with Customers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/RevenueRecognitionfromContractswithCustomersDetails</Role>
      <ShortName>Revenue Recognition from Contracts with Customers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/RevenueRecognitionfromContractswithCustomersTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="msex-20250930.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://middlesexwater.com/role/IncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://middlesexwater.com/role/IncomeTaxes</ParentRole>
      <Position>48</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="10-Q" isDefinitelyFs="true" isUsgaap="true" original="msex-20250930.htm">msex-20250930.htm</File>
    <File>msex-20250930.xsd</File>
    <File>msex-20250930_cal.xml</File>
    <File>msex-20250930_def.xml</File>
    <File>msex-20250930_lab.xml</File>
    <File>msex-20250930_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="835">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
