<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.22.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>165</ContextCount>
  <ElementCount>210</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>38</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="ctmx-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://cytomx.com/20220630/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - CONDENSED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_StatementCONDENSEDBALANCESHEETS</Role>
      <ShortName>CONDENSED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100030 - Statement - CONDENSED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_StatementCONDENSEDBALANCESHEETSParenthetical</Role>
      <ShortName>CONDENSED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100040 - Statement - CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_StatementCONDENSEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS</Role>
      <ShortName>CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100050 - Statement - CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/StatementCondensedStatementsOfStockholdersEquity</Role>
      <ShortName>CONDENSED STATEMENTS OF STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100060 - Statement - CONDENSED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_StatementCONDENSEDSTATEMENTSOFCASHFLOWS</Role>
      <ShortName>CONDENSED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100070 - Disclosure - Description of the Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureDescriptionOfBusiness</Role>
      <ShortName>Description of the Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100080 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>100090 - Disclosure - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShare</Role>
      <ShortName>Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>100100 - Disclosure - Fair Value Measurements and Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestments</Role>
      <ShortName>Fair Value Measurements and Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>100110 - Disclosure - Accrued Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureAccruedLiabilities</Role>
      <ShortName>Accrued Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>100120 - Disclosure - Research and Collaboration Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreements</Role>
      <ShortName>Research and Collaboration Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>100140 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>100150 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>100160 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureIncomeTaxes1</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>100170 - Disclosure - Subsequent Event</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureSubsequentEvent</Role>
      <ShortName>Subsequent Event</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>100180 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>17</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>100190 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>100200 - Disclosure - Net Loss Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShareTables</Role>
      <ShortName>Net Loss Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShare</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>100210 - Disclosure - Fair Value Measurements and Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestmentsTables</Role>
      <ShortName>Fair Value Measurements and Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestments</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>100220 - Disclosure - Accrued Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureAccruedLiabilitiesTables</Role>
      <ShortName>Accrued Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureAccruedLiabilities</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>100230 - Disclosure -  Research and Collaboration Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsTables</Role>
      <ShortName>Research and Collaboration Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreements</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>100240 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureStockBasedCompensation</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>100250 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesSummaryOfReconciliationOfCashCashEquivalentsAndRestrictedCashDetails</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies - Summary of Reconciliation of Cash, Cash Equivalents, and Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>100260 - Disclosure - Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureNetLossPerShareSummaryOfPotentiallyDilutiveSecuritiesExcludedFromComputationOfDilutedNetLossPerShareDetails</Role>
      <ShortName>Net Loss Per Share - Summary of Potentially Dilutive Securities Excluded from Computation of Diluted Net Loss Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>100270 - Disclosure - Fair Value Measurements and Investments - Schedule of Investments Subject to Fair Value Measurements on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestmentsScheduleOfInvestmentsSubjectToFairValueMeasurementsOnARecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements and Investments - Schedule of Investments Subject to Fair Value Measurements on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>100280 - Disclosure - Fair Value Measurements and Investments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureFairValueMeasurementsAndInvestmentsAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements and Investments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>100290 - Disclosure - Accrued Liabilities - Summary of Accrued Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureAccruedLiabilitiesSummaryOfAccruedLiabilitiesDetails</Role>
      <ShortName>Accrued Liabilities - Summary of Accrued Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>100300 - Disclosure - Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsScheduleOfRevenueByCollaborationPartnersDetails</Role>
      <ShortName>Research and Collaboration Agreements - Schedule of Revenue by Collaboration Partners (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>100310 - Disclosure - Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsAbbvieIrelandUnlimitedCompanyAdditionalInformationDetails</Role>
      <ShortName>Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>100320 - Disclosure - Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsAmgenIncAdditionalInformationDetails</Role>
      <ShortName>Research and Collaboration Agreements - Amgen, Inc - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>100330 - Disclosure - Research and Collaboration Agreements - Astellas Pharma Inc - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsAstellasPharmaIncAdditionalInformationDetails</Role>
      <ShortName>Research and Collaboration Agreements - Astellas Pharma Inc - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>100340 - Disclosure - Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsBristolMyersSquibbCompanyAdditionalInformationDetails</Role>
      <ShortName>Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>100350 - Disclosure - Research and Collaboration Agreements - Summary of Contract Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsSummaryOfContractLiabilitiesDetails</Role>
      <ShortName>Research and Collaboration Agreements - Summary of Contract Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>100360 - Disclosure - Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureResearchAndCollaborationAgreementsContractLiabilitiesAdditionalInformationDetails</Role>
      <ShortName>Research and Collaboration Agreements - Contract Liabilities - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>100370 - Disclosure - Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureStockBasedCompensationScheduleOfActivitiesUnderCompanySStockOptionPlansDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Activities Under Company's Stock Option Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>100380 - Disclosure - Stock-Based Compensation - Schedule of Company's TRSU Activities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureStockbasedCompensationScheduleOfCompanysTrsuActivitiesDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Company's TRSU Activities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>100390 - Disclosure - Stock-Based Compensation - Schedule of Activities for Company's PSUs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureStockbasedCompensationScheduleOfActivitiesForCompanysPsusDetails</Role>
      <ShortName>Stock-Based Compensation - Schedule of Activities for Company's PSUs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>100400 - Disclosure - Stock-Based Compensation - Total Stock-based Compensation Recognized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/Role_DisclosureStockBasedCompensationTotalStockBasedCompensationRecognizedDetails</Role>
      <ShortName>Stock-Based Compensation - Total Stock-based Compensation Recognized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>100410 - Disclosure - Stock-Based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureStockbasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Stock-Based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>100420 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="ctmx-20220630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>100430 - Disclosure - Subsequent Event - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://cytomx.com/20220630/taxonomy/role/DisclosureSubsequentEventAdditionalInformationDetails</Role>
      <ShortName>Subsequent Event - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</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="ctmx-20220630.htm">ctmx-20220630.htm</File>
    <File>ctmx-20220630.xsd</File>
    <File>ctmx-20220630_cal.xml</File>
    <File>ctmx-20220630_def.xml</File>
    <File>ctmx-20220630_lab.xml</File>
    <File>ctmx-20220630_pre.xml</File>
    <File>ctmx-ex31_1.htm</File>
    <File>ctmx-ex31_2.htm</File>
    <File>ctmx-ex32_1.htm</File>
    <File>ctmx-ex32_2.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="438">http://fasb.org/us-gaap/2022</BaseTaxonomy>
    <BaseTaxonomy items="30">http://xbrl.sec.gov/dei/2022</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
