<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>148</ContextCount>
  <ElementCount>398</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>57</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00300 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedStatementsOfComprehensiveLoss</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00400 - Statement - CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIENCY)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedStatementsOfStockholdersEquityDeficiency</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIENCY)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00405 - Statement - CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIENCY) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedStatementsOfStockholdersEquityDeficiencyParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (DEFICIENCY) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>00500 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Nature of Business, Basis of Presentation and Liquidity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureNatureOfBusinessBasisOfPresentationAndLiquidity</Role>
      <ShortName>Nature of Business, Basis of Presentation and Liquidity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10301 - Disclosure - BeiGene Exclusive Option and License Agreement</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureBeigeneExclusiveOptionAndLicenseAgreement</Role>
      <ShortName>BeiGene Exclusive Option and License Agreement</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Property and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosurePropertyAndEquipmentNet</Role>
      <ShortName>Property and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Accrued Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureAccruedExpenses</Role>
      <ShortName>Accrued Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Warrants</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureWarrants</Role>
      <ShortName>Warrants</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Common Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureCommonStock</Role>
      <ShortName>Common Stock</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureNetLossPerShare</Role>
      <ShortName>Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11301 - Disclosure - Defined Contribution Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureDefinedContributionPlan</Role>
      <ShortName>Defined Contribution Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>11401 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureRelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>20202 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>30203 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>30303 - Disclosure - BeiGene Exclusive Option and License Agreement (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureBeigeneExclusiveOptionAndLicenseAgreementTables</Role>
      <ShortName>BeiGene Exclusive Option and License Agreement (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureBeigeneExclusiveOptionAndLicenseAgreement</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Property and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosurePropertyAndEquipmentNetTables</Role>
      <ShortName>Property and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosurePropertyAndEquipmentNet</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Accrued Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureAccruedExpensesTables</Role>
      <ShortName>Accrued Expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureAccruedExpenses</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureLeases</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Warrants (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureWarrantsTables</Role>
      <ShortName>Warrants (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureWarrants</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>30903 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureStockBasedCompensation</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureIncomeTaxes</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>31103 - Disclosure - Net Loss Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureNetLossPerShareTables</Role>
      <ShortName>Net Loss Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureNetLossPerShare</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Nature of Business, Basis of Presentation and Liquidity - Liquidity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureNatureOfBusinessBasisOfPresentationAndLiquidityLiquidityDetails</Role>
      <ShortName>Nature of Business, Basis of Presentation and Liquidity - Liquidity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureNatureOfBusinessBasisOfPresentationAndLiquidity</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Summary of Significant Accounting Policies - Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPoliciesSignificantAccountingPoliciesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>40202 - Disclosure - Summary of Significant Accounting Policies - Warrants and Fair Value of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureSummaryOfSignificantAccountingPoliciesWarrantsAndFairValueOfFinancialInstrumentsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Warrants and Fair Value of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>40301 - Disclosure - BeiGene Exclusive Option and License Agreement - Additional information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureBeigeneExclusiveOptionAndLicenseAgreementAdditionalInformationDetails</Role>
      <ShortName>BeiGene Exclusive Option and License Agreement - Additional information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40302 - Disclosure - BeiGene Exclusive Option and License Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureBeigeneExclusiveOptionAndLicenseAgreementDetails</Role>
      <ShortName>BeiGene Exclusive Option and License Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureBeigeneExclusiveOptionAndLicenseAgreementTables</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Property and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosurePropertyAndEquipmentNetDetails</Role>
      <ShortName>Property and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosurePropertyAndEquipmentNetTables</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Accrued Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureAccruedExpensesDetails</Role>
      <ShortName>Accrued Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureAccruedExpensesTables</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureLeasesDetails</Role>
      <ShortName>Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureLeasesTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40602 - Disclosure - Leases - Future minimum lease payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureLeasesFutureMinimumLeasePaymentsDetails</Role>
      <ShortName>Leases - Future minimum lease payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Warrants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureWarrantsDetails</Role>
      <ShortName>Warrants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureWarrantsTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Common Stock - Public Offering (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureCommonStockPublicOfferingDetails</Role>
      <ShortName>Common Stock - Public Offering (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40802 - Disclosure - Common Stock - Private Placement of Common Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureCommonStockPrivatePlacementOfCommonStockDetails</Role>
      <ShortName>Common Stock - Private Placement of Common Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Stock-Based Compensation - Equity Incentive Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureStockBasedCompensationEquityIncentivePlansDetails</Role>
      <ShortName>Stock-Based Compensation - Equity Incentive Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40902 - Disclosure - Stock-Based Compensation - Stock options (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureStockBasedCompensationStockOptionsDetails</Role>
      <ShortName>Stock-Based Compensation - Stock options (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40903 - Disclosure - Stock-Based Compensation - Outstanding RSU's under the 2016 Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureStockBasedCompensationOutstandingRsuSUnder2016PlanDetails</Role>
      <ShortName>Stock-Based Compensation - Outstanding RSU's under the 2016 Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Income Taxes - Operating Loss Carryforwards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesOperatingLossCarryforwardsDetails</Role>
      <ShortName>Income Taxes - Operating Loss Carryforwards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>41002 - Disclosure - Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesDetails</Role>
      <ShortName>Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureIncomeTaxesTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>41003 - Disclosure - Income Taxes - Current and deferred expense for income tax (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesCurrentAndDeferredExpenseForIncomeTaxDetails</Role>
      <ShortName>Income Taxes - Current and deferred expense for income tax (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>41004 - Disclosure - Income Taxes - Reconciliation of Statutory and Effective Tax Rates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesReconciliationOfStatutoryAndEffectiveTaxRatesDetails</Role>
      <ShortName>Income Taxes - Reconciliation of Statutory and Effective Tax Rates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>41005 - Disclosure - Income Taxes - Components of Deferred Tax Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesComponentsOfDeferredTaxAssetsDetails</Role>
      <ShortName>Income Taxes - Components of Deferred Tax Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>41006 - Disclosure - Income Taxes - Valuation Allowance and Tax Reforms (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureIncomeTaxesValuationAllowanceAndTaxReformsDetails</Role>
      <ShortName>Income Taxes - Valuation Allowance and Tax Reforms (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Net Loss Per Share - Basic and Diluted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureNetLossPerShareBasicAndDilutedDetails</Role>
      <ShortName>Net Loss Per Share - Basic and Diluted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>41102 - Disclosure - Net Loss Per Share - Antidilutive (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureNetLossPerShareAntidilutiveDetails</Role>
      <ShortName>Net Loss Per Share - Antidilutive (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>41201 - Disclosure - Commitments and Contingencies - Manufacturing, License and Other Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureCommitmentsAndContingenciesManufacturingLicenseAndOtherAgreementsDetails</Role>
      <ShortName>Commitments and Contingencies - Manufacturing, License and Other Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>41202 - Disclosure - Commitments and Contingencies - Legal Proceedings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureCommitmentsAndContingenciesLegalProceedingsDetails</Role>
      <ShortName>Commitments and Contingencies - Legal Proceedings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>41301 - Disclosure - Defined Contribution Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureDefinedContributionPlanDetails</Role>
      <ShortName>Defined Contribution Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureDefinedContributionPlan</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="lptx-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>41401 - Disclosure - Related Party Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.leaptx.com/role/DisclosureRelatedPartyTransactionsDetails</Role>
      <ShortName>Related Party Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.leaptx.com/role/DisclosureRelatedPartyTransactions</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>lptx-20201231.xml</File>
    <File>lptx-20201231.xsd</File>
    <File>lptx-20201231_cal.xml</File>
    <File>lptx-20201231_def.xml</File>
    <File>lptx-20201231_lab.xml</File>
    <File>lptx-20201231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
