<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>111</ContextCount>
  <ElementCount>269</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>45</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>100000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>100010 - Statement - Condensed Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_StatementCondensedConsolidatedBalanceSheets</Role>
      <ShortName>Condensed Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>100030 - Statement - Condensed Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_StatementCondensedConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Condensed Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>100040 - Statement - Condensed Consolidated Statements of Operations and Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss</Role>
      <ShortName>Condensed Consolidated Statements of Operations and Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>100050 - Statement - Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveIncomeLossParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>100060 - Statement - Condensed Consolidated Statements of Stockholders' Equity (Deficit)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/StatementCondensedConsolidatedStatementsOfStockholdersEquityDeficit1</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity (Deficit)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>100070 - Statement - Condensed Consolidated Statements of Stockholders' Equity (Deficit) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/StatementCondensedConsolidatedStatementsOfStockholdersEquityDeficitParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Equity (Deficit) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>100080 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_StatementCondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.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>9</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.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>10</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Nature of the Business and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureNatureOfBusinessAndBasisOfPresentation</Role>
      <ShortName>Nature of the Business and Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Discontinued Operations and TSA</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureDiscontinuedOperationsAndTsa</Role>
      <ShortName>Discontinued Operations and TSA</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Property and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosurePropertyAndEquipmentNet</Role>
      <ShortName>Property and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Accrued Expenses and Other Current Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureAccruedExpensesAndOtherCurrentLiabilities</Role>
      <ShortName>Accrued Expenses and Other Current Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Common Stock and Stock-Based Awards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureCommonStockAndStockBasedAwards</Role>
      <ShortName>Common Stock and Stock-Based Awards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Net Income (Loss) per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureNetIncomeLossPerShare</Role>
      <ShortName>Net Income (Loss) per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureRelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995585 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995595 - Disclosure - Discontinued Operations and TSA (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureDiscontinuedOperationsAndTsaTables</Role>
      <ShortName>Discontinued Operations and TSA (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureDiscontinuedOperationsAndTsa</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995605 - Disclosure - Property and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosurePropertyAndEquipmentNetTables</Role>
      <ShortName>Property and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosurePropertyAndEquipmentNet</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureAccruedExpensesAndOtherCurrentLiabilitiesTables</Role>
      <ShortName>Accrued Expenses and Other Current Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureAccruedExpensesAndOtherCurrentLiabilities</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995625 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeases</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Common Stock and Stock-Based Awards (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureCommonStockAndStockBasedAwardsTables</Role>
      <ShortName>Common Stock and Stock-Based Awards (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureCommonStockAndStockBasedAwards</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Net Income (Loss) per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureNetIncomeLossPerShareTables</Role>
      <ShortName>Net Income (Loss) per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureNetIncomeLossPerShare</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSegmentReportingTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSegmentReporting</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995665 - Disclosure - Nature of the Business and Basis of Presentation - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureNatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail</Role>
      <ShortName>Nature of the Business and Basis of Presentation - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Summary of Significant Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995685 - Disclosure - Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesCashCashEquivalentsAndRestrictedCashDetails</Role>
      <ShortName>Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Discontinued Operations and TSA (Additional Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureDiscontinuedOperationsAndTsaAdditionalInformationDetails</Role>
      <ShortName>Discontinued Operations and TSA (Additional Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureDiscontinuedOperationsAndTsaTables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Discontinued Operations and TSA - Summary of Accrued Liabilities from Continuing Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureDiscontinuedOperationsAndTsaSummaryOfAccruedLiabilitiesFromContinuingOperationsDetails</Role>
      <ShortName>Discontinued Operations and TSA - Summary of Accrued Liabilities from Continuing Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995715 - Disclosure - Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetail</Role>
      <ShortName>Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Property and Equipment, Net - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosurePropertyAndEquipmentNetAdditionalInformationDetail</Role>
      <ShortName>Property and Equipment, Net - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995735 - Disclosure - Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureAccruedExpensesAndOtherCurrentLiabilitiesSummaryOfAccruedExpensesAndOtherCurrentLiabilitiesDetail</Role>
      <ShortName>Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Leases - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeasesAdditionalInformationDetails</Role>
      <ShortName>Leases - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Leases - Summary of Operating Lease Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeasesSummaryOfOperatingLeaseAssetsAndLiabilitiesDetails</Role>
      <ShortName>Leases - Summary of Operating Lease Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Leases - Summary of Lease Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeasesSummaryOfLeaseCostsDetails</Role>
      <ShortName>Leases - Summary of Lease Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Leases - Schedule of Future Payments of Operating Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureLeasesScheduleOfFuturePaymentsOfOperatingLeaseLiabilitiesDetails</Role>
      <ShortName>Leases - Schedule of Future Payments of Operating Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995785 - Disclosure - Common Stock and Stock-Based Awards - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureCommonStockAndStockbasedAwardsAdditionalInformationDetails</Role>
      <ShortName>Common Stock and Stock-Based Awards - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Common Stock and Stock-Based Awards - Summary of Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureCommonStockAndStockbasedAwardsSummaryOfStockOptionActivityDetails</Role>
      <ShortName>Common Stock and Stock-Based Awards - Summary of Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995805 - Disclosure - Common Stock and Stock-Based Awards - Stock Options - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureCommonStockAndStockbasedAwardsStockOptionsAdditionalInformationDetails</Role>
      <ShortName>Common Stock and Stock-Based Awards - Stock Options - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Common Stock and Stock-Based Awards - Summary of Restricted Stock Unit Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureCommonStockAndStockbasedAwardsSummaryOfRestrictedStockUnitActivityDetails</Role>
      <ShortName>Common Stock and Stock-Based Awards - Summary of Restricted Stock Unit Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995825 - Disclosure - Common Stock and Stock-Based Awards - Restricted Stock Units - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureCommonStockAndStockbasedAwardsRestrictedStockUnitsAdditionalInformationDetail</Role>
      <ShortName>Common Stock and Stock-Based Awards - Restricted Stock Units - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Common Stock and Stock-Based Awards - Summary of Stock Based Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureCommonStockAndStockbasedAwardsSummaryOfStockBasedCompensationExpenseDetails</Role>
      <ShortName>Common Stock and Stock-Based Awards - Summary of Stock Based Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Net Income (Loss) per Share - Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureNetIncomeLossPerShareBasicAndDilutedNetIncomeLossPerShareAttributableToCommonStockholdersDetail</Role>
      <ShortName>Net Income (Loss) per Share - Basic and Diluted Net Income (Loss) Per Share Attributable to Common Stockholders (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureNetIncomeLossPerShareTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Commitments and Contingencies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureCommitmentsAndContingenciesAdditionalInformationDetail</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureIncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Related Party Transactions - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/Role_DisclosureRelatedPartyTransactionsAdditionalInformationDetail</Role>
      <ShortName>Related Party Transactions - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Segment Reporting - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSegmentReportingAdditionalInformationDetail</Role>
      <ShortName>Segment Reporting - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Segment Reporting - Schedule of reportable segment net income (loss) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSegmentReportingScheduleOfReportableSegmentNetIncomeLossDetail</Role>
      <ShortName>Segment Reporting - Schedule of reportable segment net income (loss) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="mcrb-20250331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Segment Reporting - Schedule of reportable segment net income (loss) (Parenthetical) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://serestherapeutics.com/20250331/taxonomy/role/DisclosureSegmentReportingScheduleOfReportableSegmentNetIncomeLossParentheticalDetail</Role>
      <ShortName>Segment Reporting - Schedule of reportable segment net income (loss) (Parenthetical) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="mcrb-20250331.htm">mcrb-20250331.htm</File>
    <File>mcrb-20250331.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="433">http://fasb.org/us-gaap/2024</BaseTaxonomy>
    <BaseTaxonomy items="30">http://xbrl.sec.gov/dei/2024</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2024</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
