<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>899</ContextCount>
  <ElementCount>384</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>131</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="rand-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://www.randcapital.com/20251231/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - CONSOLIDATED STATEMENTS OF FINANCIAL POSITION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedStatementsOfFinancialPosition1</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF FINANCIAL POSITION</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75020 - Statement - CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedStatementsOfFinancialPositionParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF FINANCIAL POSITION (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75030 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75050 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedStatementsOfChangesInNetAssets</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN NET ASSETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>75060 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>75070 - Statement - CONSOLIDATED SCHEDULE OF PORTFOLIO INVESTMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedScheduleOfPortfolioInvestments1</Role>
      <ShortName>CONSOLIDATED SCHEDULE OF PORTFOLIO INVESTMENTS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>75080 - Statement - CONSOLIDATED SCHEDULE OF PORTFOLIO INVESTMENTS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedScheduleOfPortfolioInvestmentsParenthetical</Role>
      <ShortName>CONSOLIDATED SCHEDULE OF PORTFOLIO INVESTMENTS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>75090 - Statement - CONSOLIDATED SCHEDULE OF PORTFOLIO INVESTMENTS (Schedule of Percentage of Total Investments)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementConsolidatedScheduleOfPortfolioInvestmentsScheduleOfPercentageOfTotalInvestments</Role>
      <ShortName>CONSOLIDATED SCHEDULE OF PORTFOLIO INVESTMENTS (Schedule of Percentage of Total Investments)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>75100 - Statement - FINANCIAL HIGHLIGHTS SCHEDULE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/StatementFinancialHighlightsSchedule</Role>
      <ShortName>FINANCIAL HIGHLIGHTS SCHEDULE</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995467 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies1</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995477 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995487 - Disclosure - Other Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureOtherAssets1</Role>
      <ShortName>Other Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995497 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxes1</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995507 - Disclosure - Senior Secured Revolving Credit Facility</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacility</Role>
      <ShortName>Senior Secured Revolving Credit Facility</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995517 - Disclosure - Stockholders' Equity (Net Assets)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssets1</Role>
      <ShortName>Stockholders' Equity (Net Assets)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995527 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995537 - Disclosure - Quarterly Operations and Earnings Data - Unaudited</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureQuarterlyOperationsAndEarningsDataUnaudited1</Role>
      <ShortName>Quarterly Operations and Earnings Data - Unaudited</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995557 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureRelatedPartyTransactions1</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995567 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995577 - Disclosure - Subsequent Event</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSubsequentEvent1</Role>
      <ShortName>Subsequent Event</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995587 - Disclosure - Schedule Of Consolidated Changes In Investments At Cost And Realized Gain</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureScheduleOfConsolidatedChangesInInvestmentsAtCostAndRealizedGain1</Role>
      <ShortName>Schedule Of Consolidated Changes In Investments At Cost And Realized Gain</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995597 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies1</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995607 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies1</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995617 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestments</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995627 - Disclosure - Other Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureOtherAssetsTables</Role>
      <ShortName>Other Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureOtherAssets1</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995637 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxes1</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995647 - Disclosure - Senior Secured Revolving Credit Facility (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacilityTables</Role>
      <ShortName>Senior Secured Revolving Credit Facility (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacility</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995657 - Disclosure - Stockholders' Equity (Net Assets) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsTables</Role>
      <ShortName>Stockholders' Equity (Net Assets) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssets1</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995667 - Disclosure - Quarterly Operations and Earnings Data - Unaudited (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureQuarterlyOperationsAndEarningsDataUnauditedTables</Role>
      <ShortName>Quarterly Operations and Earnings Data - Unaudited (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureQuarterlyOperationsAndEarningsDataUnaudited1</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995677 - Disclosure - Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureRelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureRelatedPartyTransactions1</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995697 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995707 - Disclosure - Summary of Significant Accounting Policies - Schedule of Dividends Declared (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesScheduleOfDividendsDeclaredDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Schedule of Dividends Declared (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995717 - Disclosure - Summary of Significant Accounting Policies - Concentrations of Portfolio Company Values Compared to the Fair Value of the Corporation's Total Investment Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesConcentrationsOfPortfolioCompanyValuesComparedToTheFairValueOfTheCorporationsTotalInvestme</Role>
      <ShortName>Summary of Significant Accounting Policies - Concentrations of Portfolio Company Values Compared to the Fair Value of the Corporation's Total Investment Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995727 - Disclosure - Investments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestmentsAdditionalInformationDetails</Role>
      <ShortName>Investments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995737 - Disclosure - Investments - Summary of the Significant Unobservable Inputs to Determine the Fair Value of Level 3 Portfolio Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestmentsSummaryOfTheSignificantUnobservableInputsToDetermineTheFairValueOfLevel3PortfolioInvestmentsDetails</Role>
      <ShortName>Investments - Summary of the Significant Unobservable Inputs to Determine the Fair Value of Level 3 Portfolio Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995747 - Disclosure - Investments - Summary of the Components of Assets Measured at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestmentsSummaryOfTheComponentsOfAssetsMeasuredAtFairValueDetails</Role>
      <ShortName>Investments - Summary of the Components of Assets Measured at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995757 - Disclosure - Investments - Summary of Changes in Assets Measured at Fair Value Using Significant Unobservable Inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureInvestmentsSummaryOfChangesInAssetsMeasuredAtFairValueUsingSignificantUnobservableInputsDetails</Role>
      <ShortName>Investments - Summary of Changes in Assets Measured at Fair Value Using Significant Unobservable Inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995767 - Disclosure - Other Assets - Schedule of Other Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureOtherAssetsScheduleOfOtherAssetsDetails</Role>
      <ShortName>Other Assets - Schedule of Other Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995777 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995787 - Disclosure - Income Taxes - Schedule of Differences Reclassified for Tax Purposes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfDifferencesReclassifiedForTaxPurposesDetails</Role>
      <ShortName>Income Taxes - Schedule of Differences Reclassified for Tax Purposes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995797 - Disclosure - Income Taxes - Reconciliation of Net assets Resulting from Operations to Taxable Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesReconciliationOfNetAssetsResultingFromOperationsToTaxableIncomeDetails</Role>
      <ShortName>Income Taxes - Reconciliation of Net assets Resulting from Operations to Taxable Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995807 - Disclosure - Income Taxes - Schedule of Tax Character of Distributions Paid and Deemed Paid (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfTaxCharacterOfDistributionsPaidAndDeemedPaidDetails</Role>
      <ShortName>Income Taxes - Schedule of Tax Character of Distributions Paid and Deemed Paid (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995817 - Disclosure - Income Taxes - Schedule of Components of Distributable Earnings/(Accumulated Losses) and Reconciliation to Accumulated Earnings/(Deficit) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfComponentsOfDistributableEarningsaccumulatedLossesAndReconciliationToAccumulatedEarningsdeficitDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Distributable Earnings/(Accumulated Losses) and Reconciliation to Accumulated Earnings/(Deficit) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995827 - Disclosure - Income Taxes - Schedule of Net Deferred Tax Asset (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfNetDeferredTaxAssetDetails</Role>
      <ShortName>Income Taxes - Schedule of Net Deferred Tax Asset (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995837 - Disclosure - Income Taxes - Schedule of Components of Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfComponentsOfValuationAllowanceDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995847 - Disclosure - Income Taxes - Schedule of Components of Income Tax Expense (Benefit) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfComponentsOfIncomeTaxExpenseBenefitDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Income Tax Expense (Benefit) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995857 - Disclosure - Income Taxes - Summary of Income Taxes Paid (Net of Refunds) by Jurisdictions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesSummaryOfIncomeTaxesPaidNetOfRefundsByJurisdictionsDetails</Role>
      <ShortName>Income Taxes - Summary of Income Taxes Paid (Net of Refunds) by Jurisdictions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995877 - Disclosure - Income Taxes - Schedule of Reconciliation of the Expense (Benefit) From Income Taxes at the Federal Statutory Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureIncomeTaxesScheduleOfReconciliationOfTheExpenseBenefitFromIncomeTaxesAtTheFederalStatutoryRateDetails</Role>
      <ShortName>Income Taxes - Schedule of Reconciliation of the Expense (Benefit) From Income Taxes at the Federal Statutory Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995887 - Disclosure - Senior Secured Revolving Credit Facility - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacilityAdditionalInformationDetails</Role>
      <ShortName>Senior Secured Revolving Credit Facility - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995897 - Disclosure - Senior Secured Revolving Credit Facility - Schedule of Average Debt Outstanding Under the Credit Facility and Weighted Average Interest Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacilityScheduleOfAverageDebtOutstandingUnderTheCreditFacilityAndWeightedAverageInterestRateDetails</Role>
      <ShortName>Senior Secured Revolving Credit Facility - Schedule of Average Debt Outstanding Under the Credit Facility and Weighted Average Interest Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995907 - Disclosure - Senior Secured Revolving Credit Facility - Summary of Information about Senior Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacilitySummaryOfInformationAboutSeniorSecuritiesDetails</Role>
      <ShortName>Senior Secured Revolving Credit Facility - Summary of Information about Senior Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995917 - Disclosure - Senior Secured Revolving Credit Facility - Summary of Information about Corporation's Senior Securities (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSeniorSecuredRevolvingCreditFacilitySummaryOfInformationAboutCorporationsSeniorSecuritiesParentheticalDetails</Role>
      <ShortName>Senior Secured Revolving Credit Facility - Summary of Information about Corporation's Senior Securities (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995927 - Disclosure - Stockholders' Equity (Net Assets) - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsAdditionalInformationDetails</Role>
      <ShortName>Stockholders' Equity (Net Assets) - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995937 - Disclosure - Stockholders' Equity (Net Assets) -Schedule of Cash Dividend Declared (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsScheduleOfCashDividendDeclaredDetails</Role>
      <ShortName>Stockholders' Equity (Net Assets) -Schedule of Cash Dividend Declared (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995947 - Disclosure - Stockholders' Equity (Net Assets) - Schedule of Changes in Equity Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsScheduleOfChangesInEquityAccountsDetails</Role>
      <ShortName>Stockholders' Equity (Net Assets) - Schedule of Changes in Equity Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.randcapital.com/20251231/taxonomy/role/DisclosureStockholdersEquityNetAssetsTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995957 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995967 - Disclosure - Quarterly Operations and Earnings Data - Unaudited - Schedule of Quarterly Operations and Earnings Data (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureQuarterlyOperationsAndEarningsDataUnauditedScheduleOfQuarterlyOperationsAndEarningsDataDetails</Role>
      <ShortName>Quarterly Operations and Earnings Data - Unaudited - Schedule of Quarterly Operations and Earnings Data (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995987 - Disclosure - Related Party Transactions - Schedule Of Amounts Payable to RCM (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureRelatedPartyTransactionsScheduleOfAmountsPayableToRcmDetails</Role>
      <ShortName>Related Party Transactions - Schedule Of Amounts Payable to RCM (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995997 - Disclosure - Related Party Transactions - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetails</Role>
      <ShortName>Related Party Transactions - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>996007 - Disclosure - Segment Reporting - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSegmentReportingAdditionalInformationDetails</Role>
      <ShortName>Segment Reporting - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>996017 - Disclosure - Subsequent Event - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureSubsequentEventAdditionalInformationDetails</Role>
      <ShortName>Subsequent Event - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="rand-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>996027 - Disclosure - Schedule Of Consolidated Changes In Investments At Cost And Realized Gain - Schedule of changes in investments at cost and realized gain (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.randcapital.com/20251231/taxonomy/role/DisclosureScheduleOfConsolidatedChangesInInvestmentsAtCostAndRealizedGainScheduleOfChangesInInvestmentsAtCostAndRealizedGainDetails</Role>
      <ShortName>Schedule Of Consolidated Changes In Investments At Cost And Realized Gain - Schedule of changes in investments at cost and realized gain (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</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-K/A" isDefinitelyFs="true" isUsgaap="true" original="rand-20251231.htm">rand-20251231.htm</File>
    <File>rand-20251231.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1">http://fasb.org/srt/2025</BaseTaxonomy>
    <BaseTaxonomy items="2308">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="43">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
