<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>221</ContextCount>
  <ElementCount>328</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>76</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Audit Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/AuditInformation</Role>
      <ShortName>Audit Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CONSOLIDATEDBALANCESHEETS</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CONSOLIDATEDBALANCESHEETSParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF OPERATIONS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CONSOLIDATEDSTATEMENTSOFCOMPREHENSIVELOSS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0000007 - Statement - CONSOLIDATED STATEMENTS STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CONSOLIDATEDSTATEMENTSSTOCKHOLDERSEQUITY</Role>
      <ShortName>CONSOLIDATED STATEMENTS STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0000008 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWS</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0000009 - Disclosure - Organization and Description of Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/OrganizationandDescriptionofBusiness</Role>
      <ShortName>Organization and Description of Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0000010 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>0000011 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>0000012 - Disclosure - Balance Sheet Components</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/BalanceSheetComponents</Role>
      <ShortName>Balance Sheet Components</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0000013 - Disclosure - Research Collaboration and License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/ResearchCollaborationandLicenseAgreements</Role>
      <ShortName>Research Collaboration and License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>0000014 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>0000015 - Disclosure - Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquity</Role>
      <ShortName>Stockholders' Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0000016 - Disclosure - Employee Benefit Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/EmployeeBenefitPlan</Role>
      <ShortName>Employee Benefit Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0000017 - Disclosure - Restructuring and Related Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/RestructuringandRelatedActivities</Role>
      <ShortName>Restructuring and Related Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>0000018 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>0000019 - Disclosure - Subsequent Event</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SubsequentEvent</Role>
      <ShortName>Subsequent Event</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.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>21</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>9954471 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/SummaryofSignificantAccountingPolicies</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>9954472 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/SummaryofSignificantAccountingPolicies</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>9954473 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/FairValueMeasurements</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>9954474 - Disclosure - Balance Sheet Components (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/BalanceSheetComponentsTables</Role>
      <ShortName>Balance Sheet Components (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/BalanceSheetComponents</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>9954475 - Disclosure - Research Collaboration and License Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/ResearchCollaborationandLicenseAgreementsTables</Role>
      <ShortName>Research Collaboration and License Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/ResearchCollaborationandLicenseAgreements</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>9954476 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CommitmentsandContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/CommitmentsandContingencies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>9954477 - Disclosure - Stockholders' Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquityTables</Role>
      <ShortName>Stockholders' Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/StockholdersEquity</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>9954478 - Disclosure - Restructuring and Related Activities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/RestructuringandRelatedActivitiesTables</Role>
      <ShortName>Restructuring and Related Activities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/RestructuringandRelatedActivities</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>9954479 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/IncomeTaxes</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>9954480 - Disclosure - Organization and Description of Business - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/OrganizationandDescriptionofBusinessAdditionalInformationDetails</Role>
      <ShortName>Organization and Description of Business - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9954481 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPoliciesAdditionalInformationDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9954482 - Disclosure - Summary of Significant Accounting Policies - Estimated Useful Life of Asset (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPoliciesEstimatedUsefulLifeofAssetDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Estimated Useful Life of Asset (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9954483 - Disclosure - Summary of Significant Accounting Policies - Computation of Net Loss Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPoliciesComputationofNetLossPerShareDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Computation of Net Loss Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9954484 - Disclosure - Summary of Significant Accounting Policies - Potentially Dilutive Securities Not Included in Diluted Per Share Calculations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/SummaryofSignificantAccountingPoliciesPotentiallyDilutiveSecuritiesNotIncludedinDilutedPerShareCalculationsDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Potentially Dilutive Securities Not Included in Diluted Per Share Calculations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9954485 - Disclosure - Fair Value Measurements - Summary of Cash and Cash Equivalents and Marketable Securities Classified as Available-for-sale Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/FairValueMeasurementsSummaryofCashandCashEquivalentsandMarketableSecuritiesClassifiedasAvailableforsaleSecuritiesDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Cash and Cash Equivalents and Marketable Securities Classified as Available-for-sale Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9954486 - Disclosure - Fair Value Measurements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/FairValueMeasurementsAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9954487 - Disclosure - Fair Value Measurements - Summary of Available-for-sale Securities Measured at Fair Value on Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/FairValueMeasurementsSummaryofAvailableforsaleSecuritiesMeasuredatFairValueonRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Summary of Available-for-sale Securities Measured at Fair Value on Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9954488 - Disclosure - Balance Sheet Components - Schedule of Reconciliation of Cash, Cash Equivalent and Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/BalanceSheetComponentsScheduleofReconciliationofCashCashEquivalentandRestrictedCashDetails</Role>
      <ShortName>Balance Sheet Components - Schedule of Reconciliation of Cash, Cash Equivalent and Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9954489 - Disclosure - Balance Sheet Components - Schedule of Property and Equipment Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/BalanceSheetComponentsScheduleofPropertyandEquipmentNetDetails</Role>
      <ShortName>Balance Sheet Components - Schedule of Property and Equipment Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9954490 - Disclosure - Balance Sheet Components - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/BalanceSheetComponentsAdditionalInformationDetails</Role>
      <ShortName>Balance Sheet Components - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9954491 - Disclosure - Balance Sheet Components - Schedule of Accrued Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/BalanceSheetComponentsScheduleofAccruedLiabilitiesDetails</Role>
      <ShortName>Balance Sheet Components - Schedule of Accrued Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9954492 - Disclosure - Research Collaboration and License Agreements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/ResearchCollaborationandLicenseAgreementsAdditionalInformationDetails</Role>
      <ShortName>Research Collaboration and License Agreements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9954493 - Disclosure - Research Collaboration and License Agreements - Schedule of Recognized Revenue from Collaboration and License Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/ResearchCollaborationandLicenseAgreementsScheduleofRecognizedRevenuefromCollaborationandLicenseAgreementsDetails</Role>
      <ShortName>Research Collaboration and License Agreements - Schedule of Recognized Revenue from Collaboration and License Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9954494 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CommitmentsandContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9954495 - Disclosure - Commitments and Contingencies - Components of Lease Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/CommitmentsandContingenciesComponentsofLeaseCostsDetails</Role>
      <ShortName>Commitments and Contingencies - Components of Lease Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9954496 - Disclosure - Stockholders' Equity - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquityAdditionalInformationDetails</Role>
      <ShortName>Stockholders' Equity - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9954497 - Disclosure - Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Issuance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquityScheduleofSharesofCommonStockReservedforIssuanceDetails</Role>
      <ShortName>Stockholders' Equity - Schedule of Shares of Common Stock Reserved for Issuance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9954498 - Disclosure - Stockholders' Equity - Summary of Outstanding Stock Options (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquitySummaryofOutstandingStockOptionsDetails</Role>
      <ShortName>Stockholders' Equity - Summary of Outstanding Stock Options (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9954499 - Disclosure - Stockholders' Equity - Summary of Stock Based Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquitySummaryofStockBasedCompensationExpenseDetails</Role>
      <ShortName>Stockholders' Equity - Summary of Stock Based Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9954500 - Disclosure - Stockholders' Equity - Summary of Fair Value of Stock Option Granted to Employees and Directors were Estimated at Date of Grant Using Black-Scholes Option-pricing Model (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquitySummaryofFairValueofStockOptionGrantedtoEmployeesandDirectorswereEstimatedatDateofGrantUsingBlackScholesOptionpricingModelDetails</Role>
      <ShortName>Stockholders' Equity - Summary of Fair Value of Stock Option Granted to Employees and Directors were Estimated at Date of Grant Using Black-Scholes Option-pricing Model (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9954501 - Disclosure - Stockholders' Equity - Summary of Fair Value of the Rights Granted to Employees Under the 2019 ESPP was Estimated at the Date of Offer Using Black-Scholes Option-pricing Model (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/StockholdersEquitySummaryofFairValueoftheRightsGrantedtoEmployeesUnderthe2019ESPPwasEstimatedattheDateofOfferUsingBlackScholesOptionpricingModelDetails</Role>
      <ShortName>Stockholders' Equity - Summary of Fair Value of the Rights Granted to Employees Under the 2019 ESPP was Estimated at the Date of Offer Using Black-Scholes Option-pricing Model (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9954502 - Disclosure - Employee Benefit Plan - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/EmployeeBenefitPlanAdditionalInformationDetails</Role>
      <ShortName>Employee Benefit Plan - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9954503 - Disclosure - Restructuring and Related Activities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/RestructuringandRelatedActivitiesDetails</Role>
      <ShortName>Restructuring and Related Activities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.ngmbio.com/role/RestructuringandRelatedActivitiesTables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9954504 - Disclosure - Income Taxes - Schedule of Components of Losses Before Income Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxesScheduleofComponentsofLossesBeforeIncomeTaxesDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Losses Before Income Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9954505 - Disclosure - Income Taxes - Schedule of Reconciliation of Statutory U.S. Federal Rate to Effective Tax Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxesScheduleofReconciliationofStatutoryUSFederalRatetoEffectiveTaxRateDetails</Role>
      <ShortName>Income Taxes - Schedule of Reconciliation of Statutory U.S. Federal Rate to Effective Tax Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9954506 - Disclosure - Income Taxes - Schedule of Components of Net Deferred Tax Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxesScheduleofComponentsofNetDeferredTaxAssetsDetails</Role>
      <ShortName>Income Taxes - Schedule of Components of Net Deferred Tax Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9954507 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="ngm-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9954508 - Disclosure - Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.ngmbio.com/role/IncomeTaxesScheduleofReconciliationofUnrecognizedTaxBenefitsDetails</Role>
      <ShortName>Income Taxes - Schedule of Reconciliation of Unrecognized Tax Benefits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</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="ngm-20231231.htm">ngm-20231231.htm</File>
    <File>ngm-20231231.xsd</File>
    <File>ngm-20231231_cal.xml</File>
    <File>ngm-20231231_def.xml</File>
    <File>ngm-20231231_lab.xml</File>
    <File>ngm-20231231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles>
    <File>ngm-20231231_g1.jpg</File>
  </SupplementalFiles>
  <BaseTaxonomies>
    <BaseTaxonomy items="726">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="38">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
