<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>268</ContextCount>
  <ElementCount>293</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>92</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>13</UnitCount>
  <MyReports>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Balance Sheets (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Consolidated Balance Sheets (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Statements of Revenues and Expenses (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ConsolidatedStatementsofRevenuesandExpensesUnaudited</Role>
      <ShortName>Consolidated Statements of Revenues and Expenses (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Statements of Patronage Capital and Membership Fees (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ConsolidatedStatementsofPatronageCapitalandMembershipFeesUnaudited</Role>
      <ShortName>Consolidated Statements of Patronage Capital and Membership Fees (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ConsolidatedStatementsofCashFlowsUnaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952155 - Disclosure - General</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/General</Role>
      <ShortName>General</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952156 - Disclosure - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/FairValue</Role>
      <ShortName>Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952157 - Disclosure - Derivative Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DerivativeInstruments</Role>
      <ShortName>Derivative Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952158 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/InvestmentSecurities</Role>
      <ShortName>Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - Recently Issued or Adopted Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RecentlyIssuedorAdoptedAccountingPronouncements</Role>
      <ShortName>Recently Issued or Adopted Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Revenue Recognition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RevenueRecognition</Role>
      <ShortName>Revenue Recognition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Contingencies and Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ContingenciesandRegulatoryMatters</Role>
      <ShortName>Contingencies and Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Restricted Cash and Short-Term Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RestrictedCashandShortTermInvestments</Role>
      <ShortName>Restricted Cash and Short-Term Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Regulatory Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RegulatoryAssetsandLiabilities</Role>
      <ShortName>Regulatory Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Member Power Bill Prepayments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/MemberPowerBillPrepayments</Role>
      <ShortName>Member Power Bill Prepayments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Measurement of Credit Losses on Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/MeasurementofCreditLossesonFinancialInstruments</Role>
      <ShortName>Measurement of Credit Losses on Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Asset Retirement Obligations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/AssetRetirementObligations</Role>
      <ShortName>Asset Retirement Obligations</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Smarr Combined Cycle Project</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/SmarrCombinedCycleProject</Role>
      <ShortName>Smarr Combined Cycle Project</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Talbot Combustion Turbine Unit No. 7</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/TalbotCombustionTurbineUnitNo7</Role>
      <ShortName>Talbot Combustion Turbine Unit No. 7</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - Spent Nuclear Fuel Storage Costs</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/SpentNuclearFuelStorageCosts</Role>
      <ShortName>Spent Nuclear Fuel Storage Costs</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - Reportable Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ReportableSegmentInformation</Role>
      <ShortName>Reportable Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.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>24</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - General (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/GeneralPolicies</Role>
      <ShortName>General (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.opc.com/role/RecentlyIssuedorAdoptedAccountingPronouncements</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/FairValueTables</Role>
      <ShortName>Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/FairValue</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - Derivative Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DerivativeInstrumentsTables</Role>
      <ShortName>Derivative Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/DerivativeInstruments</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/InvestmentSecurities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Revenue Recognition (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RevenueRecognitionTables</Role>
      <ShortName>Revenue Recognition (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/RevenueRecognition</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/Leases</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Restricted Cash and Short-Term Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RestrictedCashandShortTermInvestmentsTables</Role>
      <ShortName>Restricted Cash and Short-Term Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/RestrictedCashandShortTermInvestments</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Regulatory Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RegulatoryAssetsandLiabilitiesTables</Role>
      <ShortName>Regulatory Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.opc.com/role/RegulatoryAssetsandLiabilities</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - General (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/GeneralDetails</Role>
      <ShortName>General (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.opc.com/role/GeneralPolicies</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Fair Value - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/FairValueScheduleofAssetsandLiabilitiesMeasuredatFairValueonaRecurringBasisDetails</Role>
      <ShortName>Fair Value - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Fair Value -Schedule of Estimated Fair Values of Long-term Debt, Including Current Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/FairValueScheduleofEstimatedFairValuesofLongtermDebtIncludingCurrentMaturitiesDetails</Role>
      <ShortName>Fair Value -Schedule of Estimated Fair Values of Long-term Debt, Including Current Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Derivative Instruments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DerivativeInstrumentsNarrativeDetails</Role>
      <ShortName>Derivative Instruments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Derivative Instruments - Schedule of Notional Volume of Natural Gas Derivatives that is Expected to Settle or Mature Each Year (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DerivativeInstrumentsScheduleofNotionalVolumeofNaturalGasDerivativesthatisExpectedtoSettleorMatureEachYearDetails</Role>
      <ShortName>Derivative Instruments - Schedule of Notional Volume of Natural Gas Derivatives that is Expected to Settle or Mature Each Year (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Derivative Instruments - Schedule of Fair Value of Derivative Instruments Not Designated as Hedging (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DerivativeInstrumentsScheduleofFairValueofDerivativeInstrumentsNotDesignatedasHedgingDetails</Role>
      <ShortName>Derivative Instruments - Schedule of Fair Value of Derivative Instruments Not Designated as Hedging (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Derivative Instruments - Schedule of Realized and Unrealized Gains (Losses) on Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DerivativeInstrumentsScheduleofRealizedandUnrealizedGainsLossesonDerivativeInstrumentsDetails</Role>
      <ShortName>Derivative Instruments - Schedule of Realized and Unrealized Gains (Losses) on Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Investment Securities - Schedule of Debt and Equity Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/InvestmentSecuritiesScheduleofDebtandEquitySecuritiesDetails</Role>
      <ShortName>Investment Securities - Schedule of Debt and Equity Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - Investment Securities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/InvestmentSecuritiesNarrativeDetails</Role>
      <ShortName>Investment Securities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - Revenue Recognition - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RevenueRecognitionNarrativeDetails</Role>
      <ShortName>Revenue Recognition - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Revenue Recognition - Schedule of Sales to Members and Sales to Non-Members (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RevenueRecognitionScheduleofSalestoMembersandSalestoNonMembersDetails</Role>
      <ShortName>Revenue Recognition - Schedule of Sales to Members and Sales to Non-Members (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Leases - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesNarrativeDetails</Role>
      <ShortName>Leases - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Leases - Schedule of Lease and Non-lease Components for all Lease Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesScheduleofLeaseandNonleaseComponentsforallLeaseAgreementsDetails</Role>
      <ShortName>Leases - Schedule of Lease and Non-lease Components for all Lease Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Leases - Schedule of Lease Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesScheduleofLeaseCostDetails</Role>
      <ShortName>Leases - Schedule of Lease Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Leases - Schedule of Cash Paid for Amounts Included in the Measurement of Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesScheduleofCashPaidforAmountsIncludedintheMeasurementofLeaseLiabilitiesDetails</Role>
      <ShortName>Leases - Schedule of Cash Paid for Amounts Included in the Measurement of Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Leases - Schedule of Maturities of Finance Lease Liabilities and Operating Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesScheduleofMaturitiesofFinanceLeaseLiabilitiesandOperatingLeaseLiabilitiesDetails</Role>
      <ShortName>Leases - Schedule of Maturities of Finance Lease Liabilities and Operating Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Leases - Schedule of Lease Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/LeasesScheduleofLeaseIncomeDetails</Role>
      <ShortName>Leases - Schedule of Lease Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Restricted Cash and Short-Term Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RestrictedCashandShortTermInvestmentsDetails</Role>
      <ShortName>Restricted Cash and Short-Term Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.opc.com/role/RestrictedCashandShortTermInvestmentsTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Regulatory Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/RegulatoryAssetsandLiabilitiesDetails</Role>
      <ShortName>Regulatory Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.opc.com/role/RegulatoryAssetsandLiabilitiesTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Debt - Department of Energy Loan Guarantee (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DebtDepartmentofEnergyLoanGuaranteeDetails</Role>
      <ShortName>Debt - Department of Energy Loan Guarantee (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Debt - Rural Utilities Service Guaranteed Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DebtRuralUtilitiesServiceGuaranteedLoansDetails</Role>
      <ShortName>Debt - Rural Utilities Service Guaranteed Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Debt - Rural Utilities Service Empowering Rural America (New ERA) Loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DebtRuralUtilitiesServiceEmpoweringRuralAmericaNewERALoanDetails</Role>
      <ShortName>Debt - Rural Utilities Service Empowering Rural America (New ERA) Loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Debt - CFC Term Loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DebtCFCTermLoanDetails</Role>
      <ShortName>Debt - CFC Term Loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Debt - Credit Facilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/DebtCreditFacilitiesDetails</Role>
      <ShortName>Debt - Credit Facilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Smarr Combined Cycle Project (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/SmarrCombinedCycleProjectDetails</Role>
      <ShortName>Smarr Combined Cycle Project (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.opc.com/role/SmarrCombinedCycleProject</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Talbot Combustion Turbine Unit No. 7 (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/TalbotCombustionTurbineUnitNo7Details</Role>
      <ShortName>Talbot Combustion Turbine Unit No. 7 (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.opc.com/role/TalbotCombustionTurbineUnitNo7</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="opc-20260630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Reportable Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.opc.com/role/ReportableSegmentInformationDetails</Role>
      <ShortName>Reportable Segment Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.opc.com/role/ReportableSegmentInformation</ParentRole>
      <Position>59</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="opc-20260630.htm">opc-20260630.htm</File>
    <File>opc-20260630.xsd</File>
    <File>opc-20260630_cal.xml</File>
    <File>opc-20260630_def.xml</File>
    <File>opc-20260630_lab.xml</File>
    <File>opc-20260630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="616">http://fasb.org/us-gaap/2026</BaseTaxonomy>
    <BaseTaxonomy items="26">http://xbrl.sec.gov/dei/2026</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2026</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
