<?xml version="1.0" encoding="utf-8"?>
<xbrl
  xmlns="http://www.xbrl.org/2003/instance"
  xmlns:dei="http://xbrl.sec.gov/dei/2025"
  xmlns:kzr="http://kezarlifesciences.com/20260508"
  xmlns:link="http://www.xbrl.org/2003/linkbase"
  xmlns:us-gaap="http://fasb.org/us-gaap/2025"
  xmlns:xbrldi="http://xbrl.org/2006/xbrldi"
  xmlns:xlink="http://www.w3.org/1999/xlink">
    <link:schemaRef xlink:href="kzr-20260508.xsd" xlink:type="simple"/>
    <context id="c20260508to20260508">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001645666</identifier>
        </entity>
        <period>
            <startDate>2026-05-08</startDate>
            <endDate>2026-05-08</endDate>
        </period>
    </context>
    <context id="c20260508to20260508_StatementClassOfStockAxis_PreferredSharePurchaseRightsMember">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001645666</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">kzr:PreferredSharePurchaseRightsMember</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2026-05-08</startDate>
            <endDate>2026-05-08</endDate>
        </period>
    </context>
    <context id="c20260508to20260508_StatementClassOfStockAxis_CommonStockMember">
        <entity>
            <identifier scheme="http://www.sec.gov/CIK">0001645666</identifier>
            <segment>
                <xbrldi:explicitMember dimension="us-gaap:StatementClassOfStockAxis">us-gaap:CommonStockMember</xbrldi:explicitMember>
            </segment>
        </entity>
        <period>
            <startDate>2026-05-08</startDate>
            <endDate>2026-05-08</endDate>
        </period>
    </context>
    <dei:AmendmentFlag
      contextRef="c20260508to20260508"
      id="Fact_5dafb411f8364775978eb65c36574622">false</dei:AmendmentFlag>
    <dei:CurrentFiscalYearEndDate
      contextRef="c20260508to20260508"
      id="Fact_970a5ab9982544ea83a42cff6d9415c9">--12-31</dei:CurrentFiscalYearEndDate>
    <dei:EntityCentralIndexKey
      contextRef="c20260508to20260508"
      id="Fact_49e8fc791eab40dfa4ab5590998f1442">0001645666</dei:EntityCentralIndexKey>
    <dei:DocumentType
      contextRef="c20260508to20260508"
      id="Fact_91e9c85d54204e4fa88ab53e49513ff9">8-K</dei:DocumentType>
    <dei:DocumentPeriodEndDate
      contextRef="c20260508to20260508"
      id="Fact_fdcf3b5fb9bc406d83027360bb681790">2026-05-08</dei:DocumentPeriodEndDate>
    <dei:EntityRegistrantName
      contextRef="c20260508to20260508"
      id="Fact_f5b8ad1728f2468c9959925e107fc136">Kezar Life Sciences, Inc.</dei:EntityRegistrantName>
    <dei:EntityIncorporationStateCountryCode
      contextRef="c20260508to20260508"
      id="Fact_80d063d5f35b4004be1c0ec0272cc4fb">DE</dei:EntityIncorporationStateCountryCode>
    <dei:EntityFileNumber
      contextRef="c20260508to20260508"
      id="Fact_626628fec34c454f9e19f76c86d94ede">001-38542</dei:EntityFileNumber>
    <dei:EntityTaxIdentificationNumber
      contextRef="c20260508to20260508"
      id="Fact_10746c82622949bf9ba45a6626aed7ba">47-3366145</dei:EntityTaxIdentificationNumber>
    <dei:EntityAddressAddressLine1
      contextRef="c20260508to20260508"
      id="Fact_b82f855f0fe640b19df15e842c0df598">4000 Shoreline Court, Suite 300</dei:EntityAddressAddressLine1>
    <dei:EntityAddressCityOrTown
      contextRef="c20260508to20260508"
      id="Fact_5bccb25e18f742fead90039789ce273e">South San Francisco</dei:EntityAddressCityOrTown>
    <dei:EntityAddressStateOrProvince
      contextRef="c20260508to20260508"
      id="Fact_2f416984f92242858e356fc3b2526c0a">CA</dei:EntityAddressStateOrProvince>
    <dei:EntityAddressPostalZipCode
      contextRef="c20260508to20260508"
      id="Fact_c43659b88ed8458389a4e6d04e7fa159">94080</dei:EntityAddressPostalZipCode>
    <dei:CityAreaCode
      contextRef="c20260508to20260508"
      id="Fact_2dca6deeddec40fe9f2a18896209f25c">650</dei:CityAreaCode>
    <dei:LocalPhoneNumber
      contextRef="c20260508to20260508"
      id="Fact_3ee7e177aa9b4b049b06d034b70551e1">822-5600</dei:LocalPhoneNumber>
    <dei:WrittenCommunications
      contextRef="c20260508to20260508"
      id="Fact_8ee90acd33f8476ab18f970b6d83209d">false</dei:WrittenCommunications>
    <dei:SolicitingMaterial
      contextRef="c20260508to20260508"
      id="Fact_3cf7d27890394d02b6f225df7f8353c7">false</dei:SolicitingMaterial>
    <dei:PreCommencementTenderOffer
      contextRef="c20260508to20260508"
      id="Fact_dd483c1bbba941a7aaff2fca79030135">false</dei:PreCommencementTenderOffer>
    <dei:PreCommencementIssuerTenderOffer
      contextRef="c20260508to20260508"
      id="Fact_12e36e36a3d441849f4b5de02b0bf658">false</dei:PreCommencementIssuerTenderOffer>
    <dei:Security12bTitle
      contextRef="c20260508to20260508_StatementClassOfStockAxis_CommonStockMember"
      id="Fact_3b9339d7aa61456fae1cb8d6008c47cc">Common Stock, $0.001 par value per share</dei:Security12bTitle>
    <dei:TradingSymbol
      contextRef="c20260508to20260508_StatementClassOfStockAxis_CommonStockMember"
      id="Fact_b5109e67546145c1b9d03fe286027af5">KZR</dei:TradingSymbol>
    <dei:SecurityExchangeName
      contextRef="c20260508to20260508_StatementClassOfStockAxis_CommonStockMember"
      id="Fact_62304a404932466886539a00e4b983e8">NASDAQ</dei:SecurityExchangeName>
    <dei:Security12bTitle
      contextRef="c20260508to20260508_StatementClassOfStockAxis_PreferredSharePurchaseRightsMember"
      id="Fact_1254b568137f4fc3bb84a6401120a7aa">Preferred Share Purchase Rights</dei:Security12bTitle>
    <dei:SecurityExchangeName
      contextRef="c20260508to20260508_StatementClassOfStockAxis_PreferredSharePurchaseRightsMember"
      id="Fact_667703f349c740e19f59f4dbe601de6e">NASDAQ</dei:SecurityExchangeName>
    <dei:EntityEmergingGrowthCompany
      contextRef="c20260508to20260508"
      id="Fact_08e8120feaa0434d96615cc5b058d17c">false</dei:EntityEmergingGrowthCompany>
</xbrl>
