<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.22.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>204</ContextCount>
  <ElementCount>239</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>56</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Condensed Balance Sheets (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_StatementCondensedBalanceSheetsUnaudited</Role>
      <ShortName>Condensed Balance Sheets (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100030 - Statement - Condensed Balance Sheets (unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_StatementCondensedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Condensed Balance Sheets (unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100040 - Statement - Condensed Statements of Operations and Comprehensive Loss (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_StatementCondensedStatementsOfOperationsAndComprehensiveLossUnaudited</Role>
      <ShortName>Condensed Statements of Operations and Comprehensive Loss (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100050 - Statement - Condensed Statements of Stockholders' Equity (Deficit) (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/StatementCondensedStatementsOfStockholdersEquityDeficitUnaudited1</Role>
      <ShortName>Condensed Statements of Stockholders' Equity (Deficit) (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100060 - Statement - Condensed Statements of Stockholders' Equity (Deficit) (unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_StatementCondensedStatementsOfStockholdersEquityDeficitUnauditedParenthetical</Role>
      <ShortName>Condensed Statements of Stockholders' Equity (Deficit) (unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100070 - Statement - Condensed Statements of Cash Flows (unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_StatementCondensedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>Condensed Statements of Cash Flows (unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100080 - Disclosure - Organization</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureOrganization</Role>
      <ShortName>Organization</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100090 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100100 - Disclosure - Fair Value Measurement</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureFairValueMeasurement</Role>
      <ShortName>Fair Value Measurement</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100110 - Disclosure - Available-for-Sale Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureAvailableForSaleSecurities</Role>
      <ShortName>Available-for-Sale Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100120 - Disclosure - Balance Sheet Components</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureBalanceSheetComponents</Role>
      <ShortName>Balance Sheet Components</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100130 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100140 - Disclosure - Collaboration, License &amp; Clinical Supply Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/DisclosureCollaborationLicenseClinicalSupplyAgreements</Role>
      <ShortName>Collaboration, License &amp; Clinical Supply Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100150 - Disclosure - Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquity</Role>
      <ShortName>Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100160 - Disclosure - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShare</Role>
      <ShortName>Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100180 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>17</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100190 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100200 - Disclosure - Fair Value Measurement (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureFairValueMeasurementTables</Role>
      <ShortName>Fair Value Measurement (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureFairValueMeasurement</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100210 - Disclosure - Available-for-Sale Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureAvailableForSaleSecuritiesTables</Role>
      <ShortName>Available-for-Sale Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureAvailableForSaleSecurities</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100220 - Disclosure - Balance Sheet Components (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureBalanceSheetComponentsTables</Role>
      <ShortName>Balance Sheet Components (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureBalanceSheetComponents</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100230 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100240 - Disclosure - Collaboration, License &amp; Clinical Supply Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/DisclosureCollaborationLicenseClinicalSupplyAgreementsTables</Role>
      <ShortName>Collaboration, License &amp; Clinical Supply Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/DisclosureCollaborationLicenseClinicalSupplyAgreements</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100250 - Disclosure - Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquityTables</Role>
      <ShortName>Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquity</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100260 - Disclosure - Net Loss Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShareTables</Role>
      <ShortName>Net Loss Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShare</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100270 - Disclosure - Organization - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureOrganizationAdditionalInformationDetails</Role>
      <ShortName>Organization - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100280 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100290 - Disclosure - Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesScheduleOfReconciliationOfCashAndCashEquivalentsAndRestrictedCashDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Schedule of Reconciliation of Cash and Cash Equivalents and Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100300 - Disclosure - Fair Value Measurement - Summary of Financial Assets Measured at Fair Value and Fair Value Hierarchy of Valuation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureFairValueMeasurementSummaryOfFinancialAssetsMeasuredAtFairValueAndFairValueHierarchyOfValuationDetails</Role>
      <ShortName>Fair Value Measurement - Summary of Financial Assets Measured at Fair Value and Fair Value Hierarchy of Valuation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100310 - Disclosure - Fair Value Measurement - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureFairValueMeasurementAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurement - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100320 - Disclosure - Available-for-Sale Securities - Schedule of Available-for-Sale Securities at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureAvailableForSaleSecuritiesScheduleOfAvailableForSaleSecuritiesAtFairValueDetails</Role>
      <ShortName>Available-for-Sale Securities - Schedule of Available-for-Sale Securities at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100330 - Disclosure - Available-for-Sale Securities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureAvailableForSaleSecuritiesAdditionalInformationDetail</Role>
      <ShortName>Available-for-Sale Securities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100340 - Disclosure - Balance Sheet Components - Property and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureBalanceSheetComponentsPropertyAndEquipmentNetDetails</Role>
      <ShortName>Balance Sheet Components - Property and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100350 - Disclosure - Balance Sheet Components - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureBalanceSheetComponentsAdditionalInformationDetails</Role>
      <ShortName>Balance Sheet Components - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100360 - Disclosure - Balance Sheet Components - Accrued Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureBalanceSheetComponentsAccruedLiabilitiesDetails</Role>
      <ShortName>Balance Sheet Components - Accrued Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100370 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100380 - Disclosure - Commitments and Contingencies - Summary of Operating Leases in Condensed Balance Sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesSummaryOfOperatingLeasesInCondensedBalanceSheetsDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Operating Leases in Condensed Balance Sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100390 - Disclosure - Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Tizona Lease and Cove Lease (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesScheduleOfFutureMinimumLeasePaymentsUnderTizonaLeaseAndCoveLeaseDetails</Role>
      <ShortName>Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Tizona Lease and Cove Lease (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100400 - Disclosure - Collaboration, License &amp; Clinical Supply Agreements - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/DisclosureCollaborationLicenseClinicalSupplyAgreementsAdditionalInformationDetail</Role>
      <ShortName>Collaboration, License &amp; Clinical Supply Agreements - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100410 - Disclosure - Collaboration, License &amp; Clinical Supply Agreements - Schedule of Collaboration and License Revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/DisclosureCollaborationLicenseClinicalSupplyAgreementsScheduleOfCollaborationAndLicenseRevenueDetails</Role>
      <ShortName>Collaboration, License &amp; Clinical Supply Agreements - Schedule of Collaboration and License Revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100420 - Disclosure - Equity - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquityAdditionalInformationDetails</Role>
      <ShortName>Equity - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100430 - Disclosure - Equity - Summary of Option Activity Under Equity Incentive Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquitySummaryOfOptionActivityUnderEquityIncentivePlanDetails</Role>
      <ShortName>Equity - Summary of Option Activity Under Equity Incentive Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>100440 - Disclosure - Equity - Summary of Fair Value of Employee and Director Stock Option Awards using a Black-Scholes Option-Pricing Model (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquitySummaryOfFairValueOfEmployeeAndDirectorStockOptionAwardsUsingBlackScholesOptionPricingModelDetails</Role>
      <ShortName>Equity - Summary of Fair Value of Employee and Director Stock Option Awards using a Black-Scholes Option-Pricing Model (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>100450 - Disclosure - Equity - Total Stock-based Compensation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureEquityTotalStockBasedCompensationDetails</Role>
      <ShortName>Equity - Total Stock-based Compensation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="harp-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>100460 - Disclosure - Net Loss Per Share - Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.harpoontx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShareSummaryOfOutstandingPotentiallyDilutiveCommonStockEquivalentsExcludedFromCalculationOfDilutedNetLossPerShareDetails</Role>
      <ShortName>Net Loss Per Share - Summary of Outstanding Potentially Dilutive Common Stock Equivalents Excluded from Calculation of Diluted Net Loss Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</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" original="harp-20220630.htm">harp-20220630.htm</File>
    <File>harp-20220630.xsd</File>
    <File>harp-20220630_cal.xml</File>
    <File>harp-20220630_def.xml</File>
    <File>harp-20220630_lab.xml</File>
    <File>harp-20220630_pre.xml</File>
    <File>harp-ex10_1.htm</File>
    <File>harp-ex31_1.htm</File>
    <File>harp-ex31_2.htm</File>
    <File>harp-ex32_1.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="564">http://fasb.org/us-gaap/2022</BaseTaxonomy>
    <BaseTaxonomy items="31">http://xbrl.sec.gov/dei/2022</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
