<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.23.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>240</ContextCount>
  <ElementCount>314</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>67</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>1001 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1002 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1003 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1004 - Statement - Consolidated Statements of Operations and Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncomeLoss</Role>
      <ShortName>Consolidated Statements of Operations and Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1005 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Deficit)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityDeficit</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity (Deficit)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1006 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1007 - Disclosure - Description of the Business</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DescriptionOfTheBusiness</Role>
      <ShortName>Description of the Business</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1008 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1009 - Disclosure - Marketable Securities and Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/MarketableSecuritiesAndInvestments</Role>
      <ShortName>Marketable Securities and Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>1010 - Disclosure - Net (Loss) Income Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/NetLossIncomePerShare</Role>
      <ShortName>Net (Loss) Income Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>1011 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>1012 - Disclosure - Property and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/PropertyAndEquipment</Role>
      <ShortName>Property and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>1013 - Disclosure - Accrued Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/AccruedExpenses</Role>
      <ShortName>Accrued Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>1014 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>1015 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/Debt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>1016 - Disclosure - Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/Equity</Role>
      <ShortName>Equity</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>1017 - Disclosure - Stock Incentive Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlans</Role>
      <ShortName>Stock Incentive Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>1018 - Disclosure - Revenue Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RevenueArrangements</Role>
      <ShortName>Revenue Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>1019 - Disclosure - Related-party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related-party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>1020 - Disclosure - Collaboration and License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreements</Role>
      <ShortName>Collaboration and License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>1021 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>1022 - Disclosure - Defined Contribution Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DefinedContributionPlan</Role>
      <ShortName>Defined Contribution Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>1023 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>1024 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>1025 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/SummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>1026 - Disclosure - Marketable Securities and Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/MarketableSecuritiesAndInvestmentsTables</Role>
      <ShortName>Marketable Securities and Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/MarketableSecuritiesAndInvestments</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>1027 - Disclosure - Net (Loss) Income Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/NetLossIncomePerShareTables</Role>
      <ShortName>Net (Loss) Income Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/NetLossIncomePerShare</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>1028 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/FairValueMeasurements</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>1029 - Disclosure - Property and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/PropertyAndEquipmentTables</Role>
      <ShortName>Property and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/PropertyAndEquipment</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>1030 - Disclosure - Accrued Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/AccruedExpensesTables</Role>
      <ShortName>Accrued Expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/AccruedExpenses</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>1031 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/Leases</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>1032 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/Debt</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>1033 - Disclosure - Equity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/EquityTables</Role>
      <ShortName>Equity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/Equity</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>1034 - Disclosure - Stock Incentive Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansTables</Role>
      <ShortName>Stock Incentive Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/StockIncentivePlans</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>1035 - Disclosure - Revenue Arrangements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RevenueArrangementsTables</Role>
      <ShortName>Revenue Arrangements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/RevenueArrangements</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>1036 - Disclosure - Related-party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RelatedPartyTransactionsTables</Role>
      <ShortName>Related-party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/RelatedPartyTransactions</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>1037 - Disclosure - Description of the Business (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DescriptionOfTheBusinessDetails</Role>
      <ShortName>Description of the Business (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/DescriptionOfTheBusiness</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>1038 - Disclosure - Marketable Securities and Investments - Summary of Available-for-Sale Marketable Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/MarketableSecuritiesAndInvestmentsSummaryOfAvailableForSaleMarketableSecuritiesDetails</Role>
      <ShortName>Marketable Securities and Investments - Summary of Available-for-Sale Marketable Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>1039 - Disclosure - Marketable Securities and Investments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/MarketableSecuritiesAndInvestmentsNarrativeDetails</Role>
      <ShortName>Marketable Securities and Investments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>1040 - Disclosure - Net (Loss) Income Per Share - Computation of Basic and Diluted Net Income (Loss) Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/NetLossIncomePerShareComputationOfBasicAndDilutedNetIncomeLossPerShareDetails</Role>
      <ShortName>Net (Loss) Income Per Share - Computation of Basic and Diluted Net Income (Loss) Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/NetLossIncomePerShareTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>1041 - Disclosure - Net (Loss) Income Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/NetLossIncomePerShareScheduleOfPotentialCommonSharesIssuableUponConversionOfWarrantsDetails</Role>
      <ShortName>Net (Loss) Income Per Share - Schedule of Potential Common Shares Issuable Upon Conversion of Warrants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/NetLossIncomePerShareTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>1042 - Disclosure - Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurementsScheduleOfAssetsMeasuredAtFairValueOnARecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>1043 - Disclosure - Fair Value Measurements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurementsNarrativeDetails</Role>
      <ShortName>Fair Value Measurements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>1044 - Disclosure - Fair Value Measurements - Schedule of Cash and Cash Equivalents (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurementsScheduleOfCashAndCashEquivalentsDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Cash and Cash Equivalents (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>1045 - Disclosure - Fair Value Measurements - Assumptions Used to Record the Fair Value of the Warrants (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurementsAssumptionsUsedToRecordTheFairValueOfTheWarrantsDetails</Role>
      <ShortName>Fair Value Measurements - Assumptions Used to Record the Fair Value of the Warrants (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>1046 - Disclosure - Fair Value Measurements - Change in the Warrant Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/FairValueMeasurementsChangeInTheWarrantLiabilitiesDetails</Role>
      <ShortName>Fair Value Measurements - Change in the Warrant Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>1047 - Disclosure - Property and Equipment - Schedule of Property and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/PropertyAndEquipmentScheduleOfPropertyAndEquipmentDetails</Role>
      <ShortName>Property and Equipment - Schedule of Property and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>1048 - Disclosure - Property and Equipment - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/PropertyAndEquipmentNarrativeDetails</Role>
      <ShortName>Property and Equipment - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>1049 - Disclosure - Accrued Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/AccruedExpensesDetails</Role>
      <ShortName>Accrued Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/AccruedExpensesTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>1050 - Disclosure - Leases - Components of Lease Costs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/LeasesComponentsOfLeaseCostsDetails</Role>
      <ShortName>Leases - Components of Lease Costs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>1051 - Disclosure - Leases - Operating Lease, Liability, Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/LeasesOperatingLeaseLiabilityMaturityDetails</Role>
      <ShortName>Leases - Operating Lease, Liability, Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>1052 - Disclosure - Leases - Other Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/LeasesOtherInformationDetails</Role>
      <ShortName>Leases - Other Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>1053 - Disclosure - Leases - Lease Term and Discount Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/LeasesLeaseTermAndDiscountRateDetails</Role>
      <ShortName>Leases - Lease Term and Discount Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>1054 - Disclosure - Leases - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/LeasesAdditionalInformationDetails</Role>
      <ShortName>Leases - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>1055 - Disclosure - Debt - Term Loans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DebtTermLoansNarrativeDetails</Role>
      <ShortName>Debt - Term Loans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>1056 - Disclosure - Debt - Term Loan And Unamortized Debt Discount Balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DebtTermLoanAndUnamortizedDebtDiscountBalancesDetails</Role>
      <ShortName>Debt - Term Loan And Unamortized Debt Discount Balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>1057 - Disclosure - Debt - Schedule of Future Minimum Payments on the Term Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DebtScheduleOfFutureMinimumPaymentsOnTheTermLoansDetails</Role>
      <ShortName>Debt - Schedule of Future Minimum Payments on the Term Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>1058 - Disclosure - Equity - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/EquityNarrativeDetails</Role>
      <ShortName>Equity - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>1059 - Disclosure - Equity - Warranty Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/EquityWarrantyActivityDetails</Role>
      <ShortName>Equity - Warranty Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>1060 - Disclosure - Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/EquityScheduleOfAuthorizedSharesOfCommonStockForFutureIssuanceDetails</Role>
      <ShortName>Equity - Schedule of Authorized Shares of Common Stock for Future Issuance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>1061 - Disclosure - Stock Incentive Plans - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansNarrativeDetails</Role>
      <ShortName>Stock Incentive Plans - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>1062 - Disclosure - Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansScheduleOfStockBasedCompensationExpenseRelatedToStockOptionsAndRestrictedCommonStockDetails</Role>
      <ShortName>Stock Incentive Plans - Schedule of Stock-Based Compensation Expense Related to Stock Options and Restricted Common Stock (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>1063 - Disclosure - Stock Incentive Plans - Schedule of Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansScheduleOfAssumptionsDetails</Role>
      <ShortName>Stock Incentive Plans - Schedule of Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>1064 - Disclosure - Stock Incentive Plans - Schedule of Options Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansScheduleOfOptionsActivityDetails</Role>
      <ShortName>Stock Incentive Plans - Schedule of Options Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>1065 - Disclosure - Stock Incentive Plans - Restricted Stock Units (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansRestrictedStockUnitsDetails</Role>
      <ShortName>Stock Incentive Plans - Restricted Stock Units (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>1066 - Disclosure - Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/StockIncentivePlansEmployeeStockPurchasePlanNarrativeDetails</Role>
      <ShortName>Stock Incentive Plans - Employee Stock Purchase Plan (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>1067 - Disclosure - Revenue Arrangements - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RevenueArrangementsNarrativeDetails</Role>
      <ShortName>Revenue Arrangements - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>1068 - Disclosure - Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RevenueArrangementsScheduleOfChangesInContractLiabilitiesDetails</Role>
      <ShortName>Revenue Arrangements - Schedule of Changes in Contract Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>1069 - Disclosure - Related-party Transactions - Number of Shares of Common Stock Purchased (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/RelatedPartyTransactionsNumberOfSharesOfCommonStockPurchasedDetails</Role>
      <ShortName>Related-party Transactions - Number of Shares of Common Stock Purchased (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>1070 - Disclosure - Collaboration and License Agreements - Ginkgo Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreementsGinkgoAgreementDetails</Role>
      <ShortName>Collaboration and License Agreements - Ginkgo Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>1071 - Disclosure - Collaboration and License Agreements - Genovis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreementsGenovisDetails</Role>
      <ShortName>Collaboration and License Agreements - Genovis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>1072 - Disclosure - Collaboration and License Agreements - Cyrus Biotechnology, Inc. (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreementsCyrusBiotechnologyIncNarrativeDetails</Role>
      <ShortName>Collaboration and License Agreements - Cyrus Biotechnology, Inc. (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>1073 - Disclosure - Collaboration and License Agreements - AskBio (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreementsAskbioNarrativeDetails</Role>
      <ShortName>Collaboration and License Agreements - AskBio (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/CollaborationAndLicenseAgreements</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>1074 - Disclosure - Collaboration and License Agreements - MIT (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreementsMitNarrativeDetails</Role>
      <ShortName>Collaboration and License Agreements - MIT (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/CollaborationAndLicenseAgreements</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>1075 - Disclosure - Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/CollaborationAndLicenseAgreementsShenyangSunshinePharmaceuticalCoLtdNarrativeDetails</Role>
      <ShortName>Collaboration and License Agreements - Shenyang Sunshine Pharmaceutical Co., Ltd (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>1076 - Disclosure - Income Taxes - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/IncomeTaxesNarrativeDetails</Role>
      <ShortName>Income Taxes - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="d536571d10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>1077 - Disclosure - Defined Contribution Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selectabio.com/role/DefinedContributionPlanDetails</Role>
      <ShortName>Defined Contribution Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selectabio.com/role/DefinedContributionPlan</ParentRole>
      <Position>77</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" original="d536571d10q.htm">d536571d10q.htm</File>
    <File>d536571dex311.htm</File>
    <File>d536571dex312.htm</File>
    <File>d536571dex321.htm</File>
    <File>selb-20230630.xsd</File>
    <File>selb-20230630_cal.xml</File>
    <File>selb-20230630_def.xml</File>
    <File>selb-20230630_lab.xml</File>
    <File>selb-20230630_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="720">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
