<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:dei="http://xbrl.sec.gov/dei/2025"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="ctmx-20260507.xsd" xlink:type="simple"/>
    <context id="C_03f54282-5251-4a84-b35c-b14452503a19">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001501989</identifier>
        </entity>
        <period>
            <startDate>2026-05-07</startDate>
            <endDate>2026-05-07</endDate>
        </period>
    </context>
    <dei:AmendmentFlag
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_b81c4a07-2311-4807-a3bc-82dea862b0d7">false</dei:AmendmentFlag>
    <dei:EntityCentralIndexKey
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_0a15c020-1604-4802-81db-4ee6a7ec72b2">0001501989</dei:EntityCentralIndexKey>
    <dei:DocumentType
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_415d91f1-2408-4441-8115-5269f988ee7f">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_cba6e92a-0bd6-4f5f-9e35-b06ff159040f">2026-05-07</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_f1b237cf-7fed-499f-8598-ed5e9660cec0">CytomX Therapeutics, Inc.</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_e1a41842-1dbc-4210-b665-1089d5a44d7e">DE</dei:EntityIncorporationStateCountryCode>
    <dei:EntityFileNumber
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_4b478b60-f4df-4344-96df-fd5ca1d6cfb6">001-37587</dei:EntityFileNumber>
    <dei:EntityTaxIdentificationNumber
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_16372738-db33-4fca-92a4-c8b46ab9b96d">27-3521219</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_13e22a77-5711-43de-a3bb-881b90094ac0">151 Oyster Point Blvd</dei:EntityAddressAddressLine1>
    <dei:EntityAddressAddressLine2
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_29aa2c07-7d49-4804-ba24-a9eb75b7bde5">Suite 400</dei:EntityAddressAddressLine2>
    <dei:EntityAddressCityOrTown
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_432213bc-0999-4c7e-827f-e8f812650ba9">South San Francisco</dei:EntityAddressCityOrTown>
    <dei:EntityAddressStateOrProvince
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_f976bef8-74c6-46aa-b7e9-256b67b08680">CA</dei:EntityAddressStateOrProvince>
    <dei:EntityAddressPostalZipCode
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_369c2b26-c022-4fc5-84b2-b79de23e9b94">94080</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_6409aa3b-5db1-4f7c-b1e8-1f4eebc2b032">650</dei:CityAreaCode>
    <dei:LocalPhoneNumber
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_42dd2e9f-3e0e-4e6e-98d9-b60d1d2d6e08">515-3185</dei:LocalPhoneNumber>
    <dei:WrittenCommunications
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_72f6f4be-8e8e-417a-9e0d-b79f32af554a">false</dei:WrittenCommunications>
    <dei:SolicitingMaterial
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_8a859d7d-7e57-40f7-a8b9-367823deee8b">false</dei:SolicitingMaterial>
    <dei:PreCommencementTenderOffer
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_ff33afab-40a5-4886-af87-1f7e8e2e2812">false</dei:PreCommencementTenderOffer>
    <dei:PreCommencementIssuerTenderOffer
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_ffa14dda-c4cc-4a1f-9e88-5396606efc96">false</dei:PreCommencementIssuerTenderOffer>
    <dei:Security12bTitle
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_6671063d-77ee-4c0b-8180-1495ff434b9e">Common Stock, $0.00001 par value per share</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_fe314224-a68f-4d29-b2c2-1ab36e33c982">CTMX</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_9997061f-4383-4fa1-ab01-d8e12e0189b0">NASDAQ</dei:SecurityExchangeName>
    <dei:EntityEmergingGrowthCompany
      contextRef="C_03f54282-5251-4a84-b35c-b14452503a19"
      id="F_13872d18-07ae-443c-9152-2316494bfff8">false</dei:EntityEmergingGrowthCompany>
</xbrl>
