<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.19.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>235</ContextCount>
  <ElementCount>326</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>99</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>10</UnitCount>
  <MyReports>
    <Report instance="ontx-20181231.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.onconova.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ontx-20181231.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.onconova.com/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ontx-20181231.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.onconova.com/role/StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ontx-20181231.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.onconova.com/role/StatementConsolidatedStatementsOfComprehensiveLoss</Role>
      <ShortName>Consolidated Statements of Comprehensive Loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00400 - Statement - Consolidated Statements of Stockholders' Equity (Deficit)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/StatementConsolidatedStatementsOfStockholdersEquityDeficit</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity (Deficit)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00500 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Nature of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusiness</Role>
      <ShortName>Nature of Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Property and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosurePropertyAndEquipment</Role>
      <ShortName>Property and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Warrants</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureWarrants</Role>
      <ShortName>Warrants</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Net Loss Per Share of Common Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNetLossPerShareOfCommonStock</Role>
      <ShortName>Net Loss Per Share of Common Stock</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Revenue</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureRevenue</Role>
      <ShortName>Revenue</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Balance Sheet Detail</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureBalanceSheetDetail</Role>
      <ShortName>Balance Sheet Detail</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Employee Benefit Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureEmployeeBenefitPlan</Role>
      <ShortName>Employee Benefit Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11301 - Disclosure - Research Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureResearchAgreements</Role>
      <ShortName>Research Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11401 - Disclosure - License and Collaboration Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureLicenseAndCollaborationAgreements</Role>
      <ShortName>License and Collaboration Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>11501 - Disclosure - Preclinical Collaboration</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosurePreclinicalCollaboration</Role>
      <ShortName>Preclinical Collaboration</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>11601 - Disclosure - Related-Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureRelatedPartyTransactions</Role>
      <ShortName>Related-Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>11701 - Disclosure - Securities Registrations and Sales Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSecuritiesRegistrationsAndSalesAgreements</Role>
      <ShortName>Securities Registrations and Sales Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>11801 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>20202 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>30203 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>30303 - Disclosure - Property and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosurePropertyAndEquipmentTables</Role>
      <ShortName>Property and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosurePropertyAndEquipment</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Warrants (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureWarrantsTables</Role>
      <ShortName>Warrants (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureWarrants</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Net Loss Per Share of Common Stock (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNetLossPerShareOfCommonStockTables</Role>
      <ShortName>Net Loss Per Share of Common Stock (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureNetLossPerShareOfCommonStock</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Revenue (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureRevenueTables</Role>
      <ShortName>Revenue (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureRevenue</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Balance Sheet Detail (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureBalanceSheetDetailTables</Role>
      <ShortName>Balance Sheet Detail (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureBalanceSheetDetail</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>30903 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureIncomeTaxes</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureStockBasedCompensation</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>31203 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Nature of Business - Reverse Stock Split (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessReverseStockSplitDetails</Role>
      <ShortName>Nature of Business - Reverse Stock Split (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40102 - Disclosure - Nature of Business - The Company - Clinical-stage Product Candidates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessCompanyClinicalStageProductCandidatesDetails</Role>
      <ShortName>Nature of Business - The Company - Clinical-stage Product Candidates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40103 - Disclosure - Nature of Business - The Company - Preclinical Collaboration (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessCompanyPreclinicalCollaborationDetails</Role>
      <ShortName>Nature of Business - The Company - Preclinical Collaboration (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40104 - Disclosure - Nature of Business - The Company (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessCompanyDetails</Role>
      <ShortName>Nature of Business - The Company (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40105 - Disclosure - Nature of Business - Liquidity - Financial Statement Items (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessLiquidityFinancialStatementItemsDetails</Role>
      <ShortName>Nature of Business - Liquidity - Financial Statement Items (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40106 - Disclosure - Nature of Business - Liquidity - Sale of Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessLiquiditySaleOfStockDetails</Role>
      <ShortName>Nature of Business - Liquidity - Sale of Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40107 - Disclosure - Nature of Business - Liquidity - Underwritten Public Offering (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessLiquidityUnderwrittenPublicOfferingDetails</Role>
      <ShortName>Nature of Business - Liquidity - Underwritten Public Offering (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40108 - Disclosure - Nature of Business - Liquidity - Rights Offering (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNatureOfBusinessLiquidityRightsOfferingDetails</Role>
      <ShortName>Nature of Business - Liquidity - Rights Offering (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Summary of Significant Accounting Policies - Unaudited Interim Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPoliciesUnauditedInterimFinancialInformationDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Unaudited Interim Financial Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40202 - Disclosure - Summary of Significant Accounting Policies - Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPoliciesSegmentInformationDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Segment Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40203 - Disclosure - Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPoliciesConcentrationsOfCreditRiskAndOffBalanceSheetRiskDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Concentrations of Credit Risk and Off-Balance Sheet Risk (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40204 - Disclosure - Summary of Significant Accounting Policies - Property and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPropertyAndEquipmentDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Property and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Property and Equipment - Property and Equipment and Related Accumulated Depreciation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosurePropertyAndEquipmentPropertyAndEquipmentAndRelatedAccumulatedDepreciationDetails</Role>
      <ShortName>Property and Equipment - Property and Equipment and Related Accumulated Depreciation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>40302 - Disclosure - Property and Equipment - Depreciation and Amortization Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosurePropertyAndEquipmentDepreciationAndAmortizationExpenseDetails</Role>
      <ShortName>Property and Equipment - Depreciation and Amortization Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Warrants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureWarrantsDetails</Role>
      <ShortName>Warrants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureWarrantsTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Net Loss Per Share of Common Stock - Computation of Basic and Diluted Earnings per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNetLossPerShareOfCommonStockComputationOfBasicAndDilutedEarningsPerShareDetails</Role>
      <ShortName>Net Loss Per Share of Common Stock - Computation of Basic and Diluted Earnings per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40502 - Disclosure - Net Loss Per Share of Common Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureNetLossPerShareOfCommonStockDetails</Role>
      <ShortName>Net Loss Per Share of Common Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureNetLossPerShareOfCommonStockTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureRevenueDetails</Role>
      <ShortName>Revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureRevenueTables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Balance Sheet Detail - Prepaid Expenses and Other Current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureBalanceSheetDetailPrepaidExpensesAndOtherCurrentAssetsDetails</Role>
      <ShortName>Balance Sheet Detail - Prepaid Expenses and Other Current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40702 - Disclosure - Balance Sheet Detail - Accrued Expenses and Other Current Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureBalanceSheetDetailAccruedExpensesAndOtherCurrentLiabilitiesDetails</Role>
      <ShortName>Balance Sheet Detail - Accrued Expenses and Other Current Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Fair Value Measurements - Securities Purchase Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsSecuritiesPurchaseAgreementDetails</Role>
      <ShortName>Fair Value Measurements - Securities Purchase Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>40802 - Disclosure - Fair Value Measurements - Rights Offering (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsRightsOfferingDetails</Role>
      <ShortName>Fair Value Measurements - Rights Offering (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>40803 - Disclosure - Fair Value Measurements - Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsFairValueAssumptionsDetails</Role>
      <ShortName>Fair Value Measurements - Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>40804 - Disclosure - Fair Value Measurements - Fair Value Hierarchy Table (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsFairValueHierarchyTableDetails</Role>
      <ShortName>Fair Value Measurements - Fair Value Hierarchy Table (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>40805 - Disclosure - Fair Value Measurements - Reconciliation of Level 3 Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsReconciliationOfLevel3LiabilitiesDetails</Role>
      <ShortName>Fair Value Measurements - Reconciliation of Level 3 Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>40806 - Disclosure - Fair Value Measurements - Transfers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureFairValueMeasurementsTransfersDetails</Role>
      <ShortName>Fair Value Measurements - Transfers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Income Taxes - Income (Loss) before Income Tax Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesIncomeLossBeforeIncomeTaxExpenseDetails</Role>
      <ShortName>Income Taxes - Income (Loss) before Income Tax Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>40902 - Disclosure - Income Taxes - Provision for Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesProvisionForIncomeTaxesDetails</Role>
      <ShortName>Income Taxes - Provision for Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>40903 - Disclosure - Income Taxes - Net Operating Loss Carryforwards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesNetOperatingLossCarryforwardsDetails</Role>
      <ShortName>Income Taxes - Net Operating Loss Carryforwards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>40904 - Disclosure - Income Taxes - Tax Credit Carry Forwards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesTaxCreditCarryForwardsDetails</Role>
      <ShortName>Income Taxes - Tax Credit Carry Forwards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>40905 - Disclosure - Income Taxes - Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Income Taxes - Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>40906 - Disclosure - Income Taxes - Deferred Tax Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesDeferredTaxAssetsDetails</Role>
      <ShortName>Income Taxes - Deferred Tax Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>40907 - Disclosure - Income Taxes - Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesValuationAllowanceDetails</Role>
      <ShortName>Income Taxes - Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>40908 - Disclosure - Income Taxes - Reconciliation of Income Tax (Expense) Benefit at the Statutory Federal Income Tax Rate and Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureIncomeTaxesReconciliationOfIncomeTaxExpenseBenefitAtStatutoryFederalIncomeTaxRateAndIncomeTaxesDetails</Role>
      <ShortName>Income Taxes - Reconciliation of Income Tax (Expense) Benefit at the Statutory Federal Income Tax Rate and Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Stock-Based Compensation - Common Stock Reserved for Issuance and Shares Available for Future Issuance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationCommonStockReservedForIssuanceAndSharesAvailableForFutureIssuanceDetails</Role>
      <ShortName>Stock-Based Compensation - Common Stock Reserved for Issuance and Shares Available for Future Issuance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>41002 - Disclosure - Stock-Based Compensation - Stock-Based Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationStockBasedCompensationExpenseDetails</Role>
      <ShortName>Stock-Based Compensation - Stock-Based Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>41003 - Disclosure - Stock-Based Compensation - Summary of Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationSummaryOfStockOptionActivityDetails</Role>
      <ShortName>Stock-Based Compensation - Summary of Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>41004 - Disclosure - Stock-Based Compensation - Stock Options Outstanding and Exercisable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationStockOptionsOutstandingAndExercisableDetails</Role>
      <ShortName>Stock-Based Compensation - Stock Options Outstanding and Exercisable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>41005 - Disclosure - Stock-Based Compensation - Options Granted after April 23, 2013 - Unrecognized Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationOptionsGrantedAfterApril232013UnrecognizedCompensationExpenseDetails</Role>
      <ShortName>Stock-Based Compensation - Options Granted after April 23, 2013 - Unrecognized Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>41006 - Disclosure - Stock-Based Compensation - Options Granted after April 23, 2013 - Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureStockBasedCompensationOptionsGrantedAfterApril232013FairValueAssumptionsDetails</Role>
      <ShortName>Stock-Based Compensation - Options Granted after April 23, 2013 - Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Employee Benefit Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureEmployeeBenefitPlanDetails</Role>
      <ShortName>Employee Benefit Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureEmployeeBenefitPlan</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>41201 - Disclosure - Commitments and Contingencies - Office and Lab Space in Newton, Pennsylvania (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureCommitmentsAndContingenciesOfficeAndLabSpaceInNewtonPennsylvaniaDetails</Role>
      <ShortName>Commitments and Contingencies - Office and Lab Space in Newton, Pennsylvania (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>41202 - Disclosure - Commitments and Contingencies - Office Space in Munich, Germany (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureCommitmentsAndContingenciesOfficeSpaceInMunichGermanyDetails</Role>
      <ShortName>Commitments and Contingencies - Office Space in Munich, Germany (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>41203 - Disclosure - Commitments and Contingencies - Future Minimum Lease Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureCommitmentsAndContingenciesFutureMinimumLeasePaymentsDetails</Role>
      <ShortName>Commitments and Contingencies - Future Minimum Lease Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>41204 - Disclosure - Commitments and Contingencies - Rent Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureCommitmentsAndContingenciesRentExpenseDetails</Role>
      <ShortName>Commitments and Contingencies - Rent Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>41301 - Disclosure - Research Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureResearchAgreementsDetails</Role>
      <ShortName>Research Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureResearchAgreements</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>41401 - Disclosure - License and Collaboration Agreements - SymBio Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureLicenseAndCollaborationAgreementsSymbioAgreementDetails</Role>
      <ShortName>License and Collaboration Agreements - SymBio Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>41501 - Disclosure - Preclinical Collaboration (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosurePreclinicalCollaborationDetails</Role>
      <ShortName>Preclinical Collaboration (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosurePreclinicalCollaboration</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>41601 - Disclosure - Related-Party Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureRelatedPartyTransactionsDetails</Role>
      <ShortName>Related-Party Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureRelatedPartyTransactions</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>41701 - Disclosure - Securities Registrations and Sales Agreements - Sales and Purchase Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSecuritiesRegistrationsAndSalesAgreementsSalesAndPurchaseAgreementsDetails</Role>
      <ShortName>Securities Registrations and Sales Agreements - Sales and Purchase Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>41702 - Disclosure - Securities Registrations and Sales Agreements - FBR capital (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSecuritiesRegistrationsAndSalesAgreementsFbrCapitalDetails</Role>
      <ShortName>Securities Registrations and Sales Agreements - FBR capital (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>41703 - Disclosure - Securities Registrations and Sales Agreements - Underwriting and Placement agency agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSecuritiesRegistrationsAndSalesAgreementsUnderwritingAndPlacementAgencyAgreementDetails</Role>
      <ShortName>Securities Registrations and Sales Agreements - Underwriting and Placement agency agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>41704 - Disclosure - Securities Registrations and Sales Agreements - underwriting agreement with HCW (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSecuritiesRegistrationsAndSalesAgreementsUnderwritingAgreementWithHcwDetails</Role>
      <ShortName>Securities Registrations and Sales Agreements - underwriting agreement with HCW (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="ontx-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>41801 - Disclosure - Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.onconova.com/role/DisclosureSubsequentEventsDetails</Role>
      <ShortName>Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.onconova.com/role/DisclosureSubsequentEvents</ParentRole>
      <Position>90</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>ontx-20181231.xml</File>
    <File>ontx-20181231.xsd</File>
    <File>ontx-20181231_cal.xml</File>
    <File>ontx-20181231_def.xml</File>
    <File>ontx-20181231_lab.xml</File>
    <File>ontx-20181231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/country/2017-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
