<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.4</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>240</ContextCount>
  <ElementCount>454</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>76</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>10</UnitCount>
  <MyReports>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>9952151 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>9952152 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>9952153 - Statement - Consolidated Statements of Operations and Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedStatementsofOperationsandComprehensiveLoss</Role>
      <ShortName>Consolidated Statements of Operations and Comprehensive Loss</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>9952154 - Statement - Consolidated Statements of Operations and Comprehensive Loss (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedStatementsofOperationsandComprehensiveLossParenthetical</Role>
      <ShortName>Consolidated Statements of Operations and Comprehensive Loss (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>9952155 - Statement - Consolidated Statements of Stockholders??? Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedStatementsofStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Stockholders??? Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>9952156 - Statement - Consolidated Statements of Stockholders??? Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedStatementsofStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Stockholders??? Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>9952157 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>9952158 - Disclosure - Organization</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/Organization</Role>
      <ShortName>Organization</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>9952159 - Disclosure - Basis of Presentation and Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/BasisofPresentationandSignificantAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>9952160 - Disclosure - Acquisitions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/Acquisitions</Role>
      <ShortName>Acquisitions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>9952161 - Disclosure - License, Collaboration and Success Payment Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LicenseCollaborationandSuccessPaymentAgreements</Role>
      <ShortName>License, Collaboration and Success Payment Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>9952162 - Disclosure - Impairment of Long-Lived Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ImpairmentofLongLivedAssets</Role>
      <ShortName>Impairment of Long-Lived Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>9952163 - Disclosure - Cash Equivalents and Marketable Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/CashEquivalentsandMarketableSecurities</Role>
      <ShortName>Cash Equivalents and Marketable Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>9952164 - Disclosure - Other Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/OtherInvestments</Role>
      <ShortName>Other Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>9952165 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>9952166 - Disclosure - Property and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/PropertyandEquipmentNet</Role>
      <ShortName>Property and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>9952167 - Disclosure - Accrued Liabilities and Other Current Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AccruedLiabilitiesandOtherCurrentLiabilities</Role>
      <ShortName>Accrued Liabilities and Other Current Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>9952168 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>9952169 - Disclosure - Convertible Preferred Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConvertiblePreferredStock</Role>
      <ShortName>Convertible Preferred Stock</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9952170 - Disclosure - Stockholders??? Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockholdersEquity</Role>
      <ShortName>Stockholders??? Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9952171 - Disclosure - Stock-based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensation</Role>
      <ShortName>Stock-based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9952172 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9952173 - Disclosure - Net Loss Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/NetLossPerShare</Role>
      <ShortName>Net Loss Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9952174 - Disclosure - Employee Benefit Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/EmployeeBenefitPlan</Role>
      <ShortName>Employee Benefit Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9952175 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9952176 - Disclosure - Related-Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related-Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9952177 - Disclosure - Segment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/Segment</Role>
      <ShortName>Segment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9952178 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995447 - Disclosure - Insider Trading Policies and Procedures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingPoliciesProc</Role>
      <ShortName>Insider Trading Policies and Procedures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995550 - Disclosure - Cybersecurity Risk Management and Strategy Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/cyd/role/CybersecurityRiskManagementAndStrategyDisclosure</Role>
      <ShortName>Cybersecurity Risk Management and Strategy Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9955511 - Disclosure - Basis of Presentation and Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/BasisofPresentationandSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.lyell.com/role/BasisofPresentationandSignificantAccountingPolicies</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9955512 - Disclosure - Acquisitions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AcquisitionsTables</Role>
      <ShortName>Acquisitions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/Acquisitions</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9955513 - Disclosure - License, Collaboration and Success Payment Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LicenseCollaborationandSuccessPaymentAgreementsTables</Role>
      <ShortName>License, Collaboration and Success Payment Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/LicenseCollaborationandSuccessPaymentAgreements</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9955514 - Disclosure - Cash Equivalents and Marketable Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/CashEquivalentsandMarketableSecuritiesTables</Role>
      <ShortName>Cash Equivalents and Marketable Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/CashEquivalentsandMarketableSecurities</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9955515 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/FairValueMeasurements</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9955516 - Disclosure - Property and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/PropertyandEquipmentNetTables</Role>
      <ShortName>Property and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/PropertyandEquipmentNet</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9955517 - Disclosure - Accrued Liabilities and Other Current Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AccruedLiabilitiesandOtherCurrentLiabilitiesTables</Role>
      <ShortName>Accrued Liabilities and Other Current Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/AccruedLiabilitiesandOtherCurrentLiabilities</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9955518 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/Leases</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9955519 - Disclosure - Stock-based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationTables</Role>
      <ShortName>Stock-based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/StockbasedCompensation</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9955520 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/IncomeTaxes</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9955521 - Disclosure - Related-Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/RelatedPartyTransactionsTables</Role>
      <ShortName>Related-Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/RelatedPartyTransactions</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9955522 - Disclosure - Segment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/SegmentTables</Role>
      <ShortName>Segment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.lyell.com/role/Segment</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9955523 - Disclosure - Basis of Presentation and Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/BasisofPresentationandSignificantAccountingPoliciesDetails</Role>
      <ShortName>Basis of Presentation and Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/BasisofPresentationandSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9955524 - Disclosure - Acquisitions - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AcquisitionsNarrativeDetails</Role>
      <ShortName>Acquisitions - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9955525 - Disclosure - Acquisitions - Schedule of Estimated Fair Value of Assets Acquired (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AcquisitionsScheduleofEstimatedFairValueofAssetsAcquiredDetails</Role>
      <ShortName>Acquisitions - Schedule of Estimated Fair Value of Assets Acquired (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9955526 - Disclosure - Acquisitions - Schedule of Consideration Paid for Merger (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AcquisitionsScheduleofConsiderationPaidforMergerDetails</Role>
      <ShortName>Acquisitions - Schedule of Consideration Paid for Merger (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9955527 - Disclosure - License, Collaboration and Success Payment Agreements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LicenseCollaborationandSuccessPaymentAgreementsNarrativeDetails</Role>
      <ShortName>License, Collaboration and Success Payment Agreements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/LicenseCollaborationandSuccessPaymentAgreementsTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9955528 - Disclosure - License, Collaboration and Success Payment Agreements - Schedule of Aggregate Potential Success Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LicenseCollaborationandSuccessPaymentAgreementsScheduleofAggregatePotentialSuccessPaymentsDetails</Role>
      <ShortName>License, Collaboration and Success Payment Agreements - Schedule of Aggregate Potential Success Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9955529 - Disclosure - Impairment of Long-Lived Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ImpairmentofLongLivedAssetsDetails</Role>
      <ShortName>Impairment of Long-Lived Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/ImpairmentofLongLivedAssets</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9955530 - Disclosure - Cash Equivalents and Marketable Securities - Schedule of Fair Value and Amortized Cost of Cash Equivalents and Marketable Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/CashEquivalentsandMarketableSecuritiesScheduleofFairValueandAmortizedCostofCashEquivalentsandMarketableSecuritiesDetails</Role>
      <ShortName>Cash Equivalents and Marketable Securities - Schedule of Fair Value and Amortized Cost of Cash Equivalents and Marketable Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9955531 - Disclosure - Cash Equivalents and Marketable Securities - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/CashEquivalentsandMarketableSecuritiesNarrativeDetails</Role>
      <ShortName>Cash Equivalents and Marketable Securities - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9955532 - Disclosure - Other Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/OtherInvestmentsDetails</Role>
      <ShortName>Other Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/OtherInvestments</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9955533 - Disclosure - Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/FairValueMeasurementsScheduleofFinancialAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9955534 - Disclosure - Fair Value Measurements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/FairValueMeasurementsNarrativeDetails</Role>
      <ShortName>Fair Value Measurements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9955535 - Disclosure - Fair Value Measurements - Schedule of SPA Put/Call is Classified as a Level 3 Financial Instrument and the Fair Value Estimated using a Monte Carlo Simulation Valuation Method (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/FairValueMeasurementsScheduleofSPAPutCallisClassifiedasaLevel3FinancialInstrumentandtheFairValueEstimatedusingaMonteCarloSimulationValuationMethodDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of SPA Put/Call is Classified as a Level 3 Financial Instrument and the Fair Value Estimated using a Monte Carlo Simulation Valuation Method (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9955536 - Disclosure - Fair Value Measurements - Schedule of Changes in Estimated Fair Value of Financial Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/FairValueMeasurementsScheduleofChangesinEstimatedFairValueofFinancialLiabilitiesDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Changes in Estimated Fair Value of Financial Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9955537 - Disclosure - Property and Equipment, Net - Schedule of Property and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/PropertyandEquipmentNetScheduleofPropertyandEquipmentNetDetails</Role>
      <ShortName>Property and Equipment, Net - Schedule of Property and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9955538 - Disclosure - Property and Equipment, Net - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/PropertyandEquipmentNetNarrativeDetails</Role>
      <ShortName>Property and Equipment, Net - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9955539 - Disclosure - Accrued Liabilities and Other Current Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/AccruedLiabilitiesandOtherCurrentLiabilitiesDetails</Role>
      <ShortName>Accrued Liabilities and Other Current Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/AccruedLiabilitiesandOtherCurrentLiabilitiesTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9955540 - Disclosure - Leases - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LeasesNarrativeDetails</Role>
      <ShortName>Leases - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9955541 - Disclosure - Leases - Schedule of Operating Lease Commitments Including Expected Lease Incentives to be Received (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LeasesScheduleofOperatingLeaseCommitmentsIncludingExpectedLeaseIncentivestobeReceivedDetails</Role>
      <ShortName>Leases - Schedule of Operating Lease Commitments Including Expected Lease Incentives to be Received (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9955542 - Disclosure - Leases - Schedule of Total Income Attributable to Sublease (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/LeasesScheduleofTotalIncomeAttributabletoSubleaseDetails</Role>
      <ShortName>Leases - Schedule of Total Income Attributable to Sublease (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9955543 - Disclosure - Convertible Preferred Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/ConvertiblePreferredStockDetails</Role>
      <ShortName>Convertible Preferred Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/ConvertiblePreferredStock</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9955544 - Disclosure - Stockholders' Equity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockholdersEquityDetails</Role>
      <ShortName>Stockholders' Equity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9955545 - Disclosure - Stock-based Compensation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationNarrativeDetails</Role>
      <ShortName>Stock-based Compensation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9955546 - Disclosure - Stock-based Compensation - Schedule of Stock-Based Compensation Expense by Classification (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationScheduleofStockBasedCompensationExpensebyClassificationDetails</Role>
      <ShortName>Stock-based Compensation - Schedule of Stock-Based Compensation Expense by Classification (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9955547 - Disclosure - Stock-based Compensation - Schedule of Performance Shares Units (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationScheduleofPerformanceSharesUnitsDetails</Role>
      <ShortName>Stock-based Compensation - Schedule of Performance Shares Units (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9955548 - Disclosure - Stock-based Compensation - Schedule of RSA and RSU Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationScheduleofRSAandRSUActivityDetails</Role>
      <ShortName>Stock-based Compensation - Schedule of RSA and RSU Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9955549 - Disclosure - Stock-based Compensation - Schedule of Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationScheduleofStockOptionActivityDetails</Role>
      <ShortName>Stock-based Compensation - Schedule of Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9955550 - Disclosure - Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model for Estimating Fair Value of Stock Options Granted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/StockbasedCompensationScheduleofAssumptionsUsedinBlackScholesOptionPricingModelforEstimatingFairValueofStockOptionsGrantedDetails</Role>
      <ShortName>Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Option-Pricing Model for Estimating Fair Value of Stock Options Granted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9955551 - Disclosure - Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesScheduleofEffectiveIncomeTaxRateReconciliationDetails</Role>
      <ShortName>Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9955552 - Disclosure - Income Taxes - Schedule of Reconciliation of Federal Statutory Income Tax Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesScheduleofReconciliationofFederalStatutoryIncomeTaxRateDetails</Role>
      <ShortName>Income Taxes - Schedule of Reconciliation of Federal Statutory Income Tax Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9955553 - Disclosure - Income Taxes - Schedule of Deferred Tax Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesScheduleofDeferredTaxAssetsDetails</Role>
      <ShortName>Income Taxes - Schedule of Deferred Tax Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9955554 - Disclosure - Income Taxes - Schedule of Changes in Company???s Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesScheduleofChangesinCompanysValuationAllowanceDetails</Role>
      <ShortName>Income Taxes - Schedule of Changes in Company???s Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9955555 - Disclosure - Income Taxes - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesNarrativeDetails</Role>
      <ShortName>Income Taxes - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9955556 - Disclosure - Income Taxes - Schedule of Changes to Company???s Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/IncomeTaxesScheduleofChangestoCompanysUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Income Taxes - Schedule of Changes to Company???s Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9955557 - Disclosure - Employee Benefit Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/EmployeeBenefitPlanDetails</Role>
      <ShortName>Employee Benefit Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/EmployeeBenefitPlan</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9955558 - Disclosure - Related-Party Transactions - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/RelatedPartyTransactionsNarrativeDetails</Role>
      <ShortName>Related-Party Transactions - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9955559 - Disclosure - Related-Party Transactions - Schedule of Total Income Attributable to Sonoma Sublease (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/RelatedPartyTransactionsScheduleofTotalIncomeAttributabletoSonomaSubleaseDetails</Role>
      <ShortName>Related-Party Transactions - Schedule of Total Income Attributable to Sonoma Sublease (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9955560 - Disclosure - Segment - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/SegmentNarrativeDetails</Role>
      <ShortName>Segment - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9955561 - Disclosure - Segment - Schedule of Reporting Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/SegmentScheduleofReportingSegmentDetails</Role>
      <ShortName>Segment - Schedule of Reporting Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="lyel-20251231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>9955562 - Disclosure - Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.lyell.com/role/SubsequentEventsDetails</Role>
      <ShortName>Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.lyell.com/role/SubsequentEvents</ParentRole>
      <Position>85</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-K" isDefinitelyFs="true" isUsgaap="true" original="lyel-20251231.htm">lyel-20251231.htm</File>
    <File>lyel-20251231.xsd</File>
    <File>lyel-20251231_cal.xml</File>
    <File>lyel-20251231_def.xml</File>
    <File>lyel-20251231_lab.xml</File>
    <File>lyel-20251231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>lyel-20251231_g1.jpg</File>
    <File>lyel-20251231_g10.jpg</File>
    <File>lyel-20251231_g11.jpg</File>
    <File>lyel-20251231_g12.jpg</File>
    <File>lyel-20251231_g13.jpg</File>
    <File>lyel-20251231_g14.jpg</File>
    <File>lyel-20251231_g15.jpg</File>
    <File>lyel-20251231_g16.jpg</File>
    <File>lyel-20251231_g17.jpg</File>
    <File>lyel-20251231_g18.jpg</File>
    <File>lyel-20251231_g19.jpg</File>
    <File>lyel-20251231_g2.jpg</File>
    <File>lyel-20251231_g3.jpg</File>
    <File>lyel-20251231_g4.jpg</File>
    <File>lyel-20251231_g5.jpg</File>
    <File>lyel-20251231_g6.jpg</File>
    <File>lyel-20251231_g7.jpg</File>
    <File>lyel-20251231_g8.jpg</File>
    <File>lyel-20251231_g9.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="856">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="15">http://xbrl.sec.gov/cyd/2025</BaseTaxonomy>
    <BaseTaxonomy items="38">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="5">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
