3.19.3.a.u2 html 202 456 1 false 68 0 false 5 false false R1.htm 100000 - Document - Document and Entity Information Sheet http://serestherapeutics.com/20191231/taxonomy/role/DocumentDocumentAndEntityInformation Document and Entity Information Cover 1 false false R2.htm 100010 - Statement - Consolidated Balance Sheets Sheet http://serestherapeutics.com/20191231/taxonomy/role/StatementConsolidatedBalanceSheets Consolidated Balance Sheets Statements 2 false false R3.htm 100020 - Statement - Consolidated Balance Sheets (Parenthetical) Sheet http://serestherapeutics.com/20191231/taxonomy/role/StatementConsolidatedBalanceSheetsParenthetical Consolidated Balance Sheets (Parenthetical) Statements 3 false false R4.htm 100030 - Statement - Consolidated Statements of Operations and Comprehensive Loss Sheet http://serestherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLoss Consolidated Statements of Operations and Comprehensive Loss Statements 4 false false R5.htm 100040 - Statement - Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) Sheet http://serestherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfOperationsAndComprehensiveLossParenthetical Consolidated Statements of Operations and Comprehensive Loss (Parenthetical) Statements 5 false false R6.htm 100050 - Statement - Consolidated Statements of Stockholders' Equity (Deficit) Sheet http://serestherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfStockholdersEquityDeficit Consolidated Statements of Stockholders' Equity (Deficit) Statements 6 false false R7.htm 100060 - Statement - Consolidated Statements of Cash Flows Sheet http://serestherapeutics.com/20191231/taxonomy/role/StatementConsolidatedStatementsOfCashFlows Consolidated Statements of Cash Flows Statements 7 false false R8.htm 100070 - Disclosure - Nature of the Business and Basis of Presentation Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNatureOfBusinessAndBasisOfPresentation Nature of the Business and Basis of Presentation Notes 8 false false R9.htm 100080 - Disclosure - Summary of Significant Accounting Policies Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies Summary of Significant Accounting Policies Notes 9 false false R10.htm 100090 - Disclosure - Fair Value of Financial Assets and Liabilities Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilities Fair Value of Financial Assets and Liabilities Notes 10 false false R11.htm 100100 - Disclosure - Investments Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureInvestments Investments Notes 11 false false R12.htm 100110 - Disclosure - Property and Equipment, Net Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNet Property and Equipment, Net Notes 12 false false R13.htm 100120 - Disclosure - Accrued Expenses and Other Current Liabilities Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities Accrued Expenses and Other Current Liabilities Notes 13 false false R14.htm 100130 - Disclosure - Leases Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeases Leases Notes 14 false false R15.htm 100140 - Disclosure - Restructuring Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRestructuring Restructuring Notes 15 false false R16.htm 100150 - Disclosure - Notes Payable Notes http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNotesPayable Notes Payable Notes 16 false false R17.htm 100160 - Disclosure - Convertible Preferred Stock Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureConvertiblePreferredStock Convertible Preferred Stock Notes 17 false false R18.htm 100170 - Disclosure - Stockholders' Equity Common Stock Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStock Stockholders' Equity Common Stock Notes 18 false false R19.htm 100180 - Disclosure - Collaboration Revenue Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCollaborationRevenue Collaboration Revenue Notes 19 false false R20.htm 100190 - Disclosure - Net Loss per Share Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShare Net Loss per Share Notes 20 false false R21.htm 100200 - Disclosure - Commitments and Contingencies Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingencies Commitments and Contingencies Notes 21 false false R22.htm 100210 - Disclosure - Income Taxes Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxes Income Taxes Notes 22 false false R23.htm 100220 - Disclosure - Selected Quarterly Financial Data (Unaudited) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnaudited Selected Quarterly Financial Data (Unaudited) Notes 23 false false R24.htm 100230 - Disclosure - Related Party Transactions Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRelatedPartyTransactions Related Party Transactions Notes 24 false false R25.htm 100240 - Disclosure - 401(k) Savings Plan Sheet http://serestherapeutics.com/20191231/taxonomy/role/Disclosure401KSavingsPlan 401(k) Savings Plan Notes 25 false false R26.htm 100250 - Disclosure - Summary of Significant Accounting Policies (Policies) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies Summary of Significant Accounting Policies (Policies) Policies http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies 26 false false R27.htm 100260 - Disclosure - Fair Value of Financial Assets and Liabilities (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesTables Fair Value of Financial Assets and Liabilities (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilities 27 false false R28.htm 100270 - Disclosure - Investments (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureInvestmentsTables Investments (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureInvestments 28 false false R29.htm 100280 - Disclosure - Property and Equipment, Net (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNetTables Property and Equipment, Net (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNet 29 false false R30.htm 100290 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesTables Accrued Expenses and Other Current Liabilities (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities 30 false false R31.htm 100300 - Disclosure - Leases (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesTables Leases (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeases 31 false false R32.htm 100310 - Disclosure - Restructuring (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRestructuringTables Restructuring (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRestructuring 32 false false R33.htm 100320 - Disclosure - Notes Payable (Tables) Notes http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNotesPayableTables Notes Payable (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNotesPayable 33 false false R34.htm 100330 - Disclosure - Stockholders' Equity Common Stock (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockTables Stockholders' Equity Common Stock (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStock 34 false false R35.htm 100340 - Disclosure - Collaboration Revenue (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCollaborationRevenueTables Collaboration Revenue (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCollaborationRevenue 35 false false R36.htm 100350 - Disclosure - Net Loss per Share (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShareTables Net Loss per Share (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShare 36 false false R37.htm 100360 - Disclosure - Income Taxes (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesTables Income Taxes (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxes 37 false false R38.htm 100370 - Disclosure - Selected Quarterly Financial Data (Unaudited) (Tables) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables Selected Quarterly Financial Data (Unaudited) (Tables) Tables http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnaudited 38 false false R39.htm 100380 - Disclosure - Nature of the Business and Basis of Presentation - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNatureOfBusinessAndBasisOfPresentationAdditionalInformationDetail Nature of the Business and Basis of Presentation - Additional Information (Detail) Details 39 false false R40.htm 100390 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesAdditionalInformationDetail Summary of Significant Accounting Policies - Additional Information (Detail) Details 40 false false R41.htm 100400 - Disclosure - Fair Value of Financial Assets and Liabilities - Summary of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesSummaryOfFairValueHierarchyForAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail Fair Value of Financial Assets and Liabilities - Summary of Fair Value Hierarchy for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) Details 41 false false R42.htm 100410 - Disclosure - Fair Value of Financial Assets and Liabilities - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureFairValueOfFinancialAssetsAndLiabilitiesAdditionalInformationDetail Fair Value of Financial Assets and Liabilities - Additional Information (Detail) Details 42 false false R43.htm 100420 - Disclosure - Investments - Schedule of Investments by Security Type (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureInvestmentsScheduleOfInvestmentsBySecurityTypeDetail Investments - Schedule of Investments by Security Type (Detail) Details 43 false false R44.htm 100430 - Disclosure - Investments - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureInvestmentsAdditionalInformationDetail Investments - Additional Information (Detail) Details 44 false false R45.htm 100440 - Disclosure - Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetail Property and Equipment, Net - Schedule of Property and Equipment, Net (Detail) Details 45 false false R46.htm 100450 - Disclosure - Property and Equipment, Net - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosurePropertyAndEquipmentNetAdditionalInformationDetail Property and Equipment, Net - Additional Information (Detail) Details 46 false false R47.htm 100460 - Disclosure - Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesSummaryOfAccruedExpensesAndOtherCurrentLiabilitiesDetail Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) Details 47 false false R48.htm 100470 - Disclosure - Leases - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesAdditionalInformationDetail Leases - Additional Information (Detail) Details 48 false false R49.htm 100480 - Disclosure - Leases - Summary of Impact on Consolidated Balance Sheets for Opening Balances Related to Lease (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesSummaryOfImpactOnConsolidatedBalanceSheetsForOpeningBalancesRelatedToLeaseDetail Leases - Summary of Impact on Consolidated Balance Sheets for Opening Balances Related to Lease (Detail) Details 49 false false R50.htm 100490 - Disclosure - Leases - Summary of Future Undiscounted Cash Inflows Under the Sublease (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesSummaryOfFutureUndiscountedCashInflowsUnderSubleaseDetail Leases - Summary of Future Undiscounted Cash Inflows Under the Sublease (Detail) Details 50 false false R51.htm 100500 - Disclosure - Leases - Summary of Operating Lease Assets and Liabilities (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesSummaryOfOperatingLeaseAssetsAndLiabilitiesDetail Leases - Summary of Operating Lease Assets and Liabilities (Detail) Details 51 false false R52.htm 100510 - Disclosure - Leases - Summary of Lease Costs (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesSummaryOfLeaseCostsDetail Leases - Summary of Lease Costs (Detail) Details 52 false false R53.htm 100520 - Disclosure - Leases - Schedule of Future Payments of Operating Lease Liabilities (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesScheduleOfFuturePaymentsOfOperatingLeaseLiabilitiesDetail Leases - Schedule of Future Payments of Operating Lease Liabilities (Detail) Details 53 false false R54.htm 100540 - Disclosure - Leases - Summary of Future Minimum Lease Payments Under Operating Leases (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureLeasesSummaryOfFutureMinimumLeasePaymentsUnderOperatingLeasesDetail Leases - Summary of Future Minimum Lease Payments Under Operating Leases (Detail) Details 54 false false R55.htm 100550 - Disclosure - Restructuring - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRestructuringAdditionalInformationDetail Restructuring - Additional Information (Detail) Details 55 false false R56.htm 100560 - Disclosure - Restructuring - Schedule of Components of Outstanding Restructuring Liabilities (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRestructuringScheduleOfComponentsOfOutstandingRestructuringLiabilitiesDetail Restructuring - Schedule of Components of Outstanding Restructuring Liabilities (Detail) Details 56 false false R57.htm 100570 - Disclosure - Notes Payable - Additional Information (Detail) Notes http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNotesPayableAdditionalInformationDetail Notes Payable - Additional Information (Detail) Details 57 false false R58.htm 100580 - Disclosure - Notes Payable - Summary of Future Principal Payments Due Under Arrangement, Excluding Interest and End of Term Charge (Detail) Notes http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNotesPayableSummaryOfFuturePrincipalPaymentsDueUnderArrangementExcludingInterestAndEndOfTermChargeDetail Notes Payable - Summary of Future Principal Payments Due Under Arrangement, Excluding Interest and End of Term Charge (Detail) Details 58 false false R59.htm 100590 - Disclosure - Convertible Preferred Stock - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureConvertiblePreferredStockAdditionalInformationDetail Convertible Preferred Stock - Additional Information (Detail) Details 59 false false R60.htm 100600 - Disclosure - Stockholders' Equity Common Stock - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockAdditionalInformationDetail Stockholders' Equity Common Stock - Additional Information (Detail) Details 60 false false R61.htm 100610 - Disclosure - Stockholders' Equity Common Stock - 2012 Stock Incentive plan - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStock2012StockIncentivePlanAdditionalInformationDetail Stockholders' Equity Common Stock - 2012 Stock Incentive plan - Additional Information (Detail) Details 61 false false R62.htm 100620 - Disclosure - Stockholders' Equity Common Stock - 2015 Incentive Award Plan - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStock2015IncentiveAwardPlanAdditionalInformationDetail Stockholders' Equity Common Stock - 2015 Incentive Award Plan - Additional Information (Detail) Details 62 false false R63.htm 100630 - Disclosure - Stockholders' Equity Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStock2015EmployeeStockPurchasePlanAdditionalInformationDetail Stockholders' Equity Common Stock - 2015 Employee Stock Purchase Plan - Additional Information (Detail) Details 63 false false R64.htm 100640 - Disclosure - Stockholders' Equity Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockSummaryOfEstimatedFairValueOfEmployeeStockOptionsDetail Stockholders' Equity Common Stock - Summary of Estimated Fair Value of Employee Stock Options (Detail) Details 64 false false R65.htm 100650 - Disclosure - Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockSummaryOfStockOptionActivityDetail Stockholders' Equity Common Stock - Summary of Stock Option Activity (Detail) Details 65 false false R66.htm 100660 - Disclosure - Stockholders' Equity Common Stock - Stock Options - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockStockOptionsAdditionalInformationDetail Stockholders' Equity Common Stock - Stock Options - Additional Information (Detail) Details 66 false false R67.htm 100670 - Disclosure - Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockSummaryOfRestrictedStockActivityDetail Stockholders' Equity Common Stock - Summary of Restricted Stock Activity (Detail) Details 67 false false R68.htm 100680 - Disclosure - Stockholders' Equity Common Stock - Restricted Stock Units - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockRestrictedStockUnitsAdditionalInformationDetail Stockholders' Equity Common Stock - Restricted Stock Units - Additional Information (Detail) Details 68 false false R69.htm 100690 - Disclosure - Stockholders' Equity Common Stock - Summary of Stock Based Compensation Expense (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockSummaryOfStockBasedCompensationExpenseDetail Stockholders' Equity Common Stock - Summary of Stock Based Compensation Expense (Detail) Details 69 false false R70.htm 100700 - Disclosure - Stockholders' Equity Common Stock - Stock Based Compensation - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureStockholdersEquityCommonStockStockBasedCompensationAdditionalInformationDetail Stockholders' Equity Common Stock - Stock Based Compensation - Additional Information (Detail) Details 70 false false R71.htm 100710 - Disclosure - Collaboration Revenue - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCollaborationRevenueAdditionalInformationDetail Collaboration Revenue - Additional Information (Detail) Details 71 false false R72.htm 100720 - Disclosure - Collaboration Revenue - Changes in Contract Liabilities (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCollaborationRevenueChangesInContractLiabilitiesDetail Collaboration Revenue - Changes in Contract Liabilities (Detail) Details 72 false false R73.htm 100730 - Disclosure - Collaboration Revenue - Schedule of Revenue Recognized (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCollaborationRevenueScheduleOfRevenueRecognizedDetail Collaboration Revenue - Schedule of Revenue Recognized (Detail) Details 73 false false R74.htm 100740 - Disclosure - Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShareBasicAndDilutedNetLossPerShareAttributableToCommonStockholdersDetail Net Loss per Share - Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) Details 74 false false R75.htm 100750 - Disclosure - Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureNetLossPerShareSummaryOfPotentialCommonSharesExcludedFromCalculationOfDilutedNetLossPerShareDetail Net Loss per Share - Summary of Potential Common Shares Excluded from Calculation of Diluted Net Loss per Share (Detail) Details 75 false false R76.htm 100760 - Disclosure - Commitments and Contingencies - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetail Commitments and Contingencies - Additional Information (Detail) Details 76 false false R77.htm 100770 - Disclosure - Income Taxes - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetail Income Taxes - Additional Information (Detail) Details 77 false false R78.htm 100780 - Disclosure - Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesScheduleOfReconciliationOfUSFederalStatutoryIncomeTaxRateToCompanySEffectiveIncomeTaxRateDetail Income Taxes - Schedule of Reconciliation of U.S Federal Statutory Income Tax Rate to Company's Effective Income Tax Rate (Detail) Details 78 false false R79.htm 100790 - Disclosure - Income Taxes - Schedule of Net Deferred Tax Assets (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesScheduleOfNetDeferredTaxAssetsDetail Income Taxes - Schedule of Net Deferred Tax Assets (Detail) Details 79 false false R80.htm 100800 - Disclosure - Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureIncomeTaxesSummaryOfChangesInValuationAllowanceForDeferredTaxAssetsDetail Income Taxes - Summary of Changes In Valuation Allowance for Deferred Tax Assets (Detail) Details 80 false false R81.htm 100810 - Disclosure - Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedSummaryOfQuarterlyFinancialInformationDetail Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Detail) Details http://serestherapeutics.com/20191231/taxonomy/role/DisclosureSelectedQuarterlyFinancialDataUnauditedTables 81 false false R82.htm 100820 - Disclosure - Related Party Transactions - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetail Related Party Transactions - Additional Information (Detail) Details 82 false false R83.htm 100830 - Disclosure - 401(k) Savings Plan - Additional Information (Detail) Sheet http://serestherapeutics.com/20191231/taxonomy/role/Disclosure401KSavingsPlanAdditionalInformationDetail 401(k) Savings Plan - Additional Information (Detail) Details 83 false false All Reports Book All Reports mcrb-20191231.xml mcrb-20191231.xsd mcrb-20191231_cal.xml mcrb-20191231_def.xml mcrb-20191231_lab.xml mcrb-20191231_pre.xml http://fasb.org/srt/2019-01-31 http://fasb.org/us-gaap/2019-01-31 http://xbrl.sec.gov/dei/2019-01-31 http://xbrl.sec.gov/stpr/2018-01-31 true true