<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.22.2.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>2619</ContextCount>
  <ElementCount>386</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>314</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>13</UnitCount>
  <MyReports>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Consolidated Statements of Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedStatementsOfAssetsAndLiabilities</Role>
      <ShortName>Consolidated Statements of Assets and Liabilities</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - Consolidated Statements of Assets and Liabilities (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedStatementsOfAssetsAndLiabilitiesParenthetical</Role>
      <ShortName>Consolidated Statements of Assets and Liabilities (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - Consolidated Statements of Operations</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedStatementsOfOperations</Role>
      <ShortName>Consolidated Statements of Operations</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00300 - Statement - Consolidated Statements of Changes in Net Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedStatementsOfChangesInNetAssets</Role>
      <ShortName>Consolidated Statements of Changes in Net Assets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00400 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00500 - Statement - Consolidated Schedule of Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedScheduleOfInvestments</Role>
      <ShortName>Consolidated Schedule of Investments</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>00505 - Statement - Consolidated Schedule of Investments - Forward Foreign Currency Exchange Contracts</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/StatementConsolidatedScheduleOfInvestmentsForwardForeignCurrencyExchangeContracts</Role>
      <ShortName>Consolidated Schedule of Investments - Forward Foreign Currency Exchange Contracts</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Organization</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureOrganization</Role>
      <ShortName>Organization</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebt</Role>
      <ShortName>Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDerivatives</Role>
      <ShortName>Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Distributions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDistributions</Role>
      <ShortName>Distributions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Common Stock Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureCommonStockCapital</Role>
      <ShortName>Common Stock Capital</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Financial Highlights</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFinancialHighlights</Role>
      <ShortName>Financial Highlights</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureSubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>20202 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>30303 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureInvestments</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureRelatedPartyTransactions</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtTables</Role>
      <ShortName>Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureDebt</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureDerivatives</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Distributions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDistributionsTables</Role>
      <ShortName>Distributions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureDistributions</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>31103 - Disclosure - Financial Highlights (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFinancialHighlightsTables</Role>
      <ShortName>Financial Highlights (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureFinancialHighlights</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Organization (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureOrganizationDetails</Role>
      <ShortName>Organization (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureOrganization</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Summary of Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureSummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Investments - Investment By Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsInvestmentByTypeDetails</Role>
      <ShortName>Investments - Investment By Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>40302 - Disclosure - Investments - Investments By Geographic Region (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsInvestmentsByGeographicRegionDetails</Role>
      <ShortName>Investments - Investments By Geographic Region (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>40303 - Disclosure - Investments - Investments By Industry (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsInvestmentsByIndustryDetails</Role>
      <ShortName>Investments - Investments By Industry (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>40304 - Disclosure - Investments - International Senior Loan Program, LLC (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsInternationalSeniorLoanProgramLlcDetails</Role>
      <ShortName>Investments - International Senior Loan Program, LLC (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>40305 - Disclosure - Investments - SLP's portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsSlpSPortfolioDetailss</Role>
      <ShortName>Investments - SLP's portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40306 - Disclosure - Investments - ISLP Schedule of Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsIslpScheduleOfInvestmentsDetails</Role>
      <ShortName>Investments - ISLP Schedule of Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40307 - Disclosure - Investments - ISLP Forward Foreign Currency Exchange Contracts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsIslpForwardForeignCurrencyExchangeContractsDetails</Role>
      <ShortName>Investments - ISLP Forward Foreign Currency Exchange Contracts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40308 - Disclosure - Investments - ISLP Balance Sheet (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsIslpBalanceSheetDetails</Role>
      <ShortName>Investments - ISLP Balance Sheet (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40309 - Disclosure - Investments - ISLP Statements of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsIslpStatementsOfOperationsDetails</Role>
      <ShortName>Investments - ISLP Statements of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40310 - Disclosure - Investments - SLP (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsSlpDetails</Role>
      <ShortName>Investments - SLP (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40311 - Disclosure - Investments - 2018-1 Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestments20181NotesDetails</Role>
      <ShortName>Investments - 2018-1 Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40312 - Disclosure - Investments - SLP's portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsSlpSPortfolioDetails</Role>
      <ShortName>Investments - SLP's portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40313 - Disclosure - Investments - SLP Schedule of Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsSlpScheduleOfInvestmentsDetails</Role>
      <ShortName>Investments - SLP Schedule of Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40314 - Disclosure - Investments - SLP Balance Sheet (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsSlpBalanceSheetDetails</Role>
      <ShortName>Investments - SLP Balance Sheet (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40315 - Disclosure - Investments - SLP Statement of Operations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureInvestmentsSlpStatementOfOperationsDetails</Role>
      <ShortName>Investments - SLP Statement of Operations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Fair Value Measurements - Schedule of investments by fair value hierarchy (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFairValueMeasurementsScheduleOfInvestmentsByFairValueHierarchyDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of investments by fair value hierarchy (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40402 - Disclosure - Fair Value Measurements - Reconciliation of investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFairValueMeasurementsReconciliationOfInvestmentsDetails</Role>
      <ShortName>Fair Value Measurements - Reconciliation of investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>40403 - Disclosure - Fair Value Measurements - Significant unobservable inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFairValueMeasurementsSignificantUnobservableInputsDetails</Role>
      <ShortName>Fair Value Measurements - Significant unobservable inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Related Party Transactions - Base Management Fee (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsBaseManagementFeeDetails</Role>
      <ShortName>Related Party Transactions - Base Management Fee (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40502 - Disclosure - Related Party Transactions - Incentive Fee (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsIncentiveFeeDetails</Role>
      <ShortName>Related Party Transactions - Incentive Fee (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40503 - Disclosure - Related Party Transactions - Administration Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsAdministrationAgreementDetails</Role>
      <ShortName>Related Party Transactions - Administration Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40504 - Disclosure - Related Party Transactions - Resource Sharing Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsResourceSharingAgreementDetails</Role>
      <ShortName>Related Party Transactions - Resource Sharing Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40505 - Disclosure - Related Party Transactions - Revolving Advisor Loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsRevolvingAdvisorLoanDetails</Role>
      <ShortName>Related Party Transactions - Revolving Advisor Loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40506 - Disclosure - Related Party Transactions - Related Party Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsRelatedPartyCommitmentsDetails</Role>
      <ShortName>Related Party Transactions - Related Party Commitments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40507 - Disclosure - Related Party Transactions - Non-Controlled Affiliate and Controlled Affiliate Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureRelatedPartyTransactionsNonControlledAffiliateAndControlledAffiliateInvestmentsDetails</Role>
      <ShortName>Related Party Transactions - Non-Controlled Affiliate and Controlled Affiliate Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtDetails</Role>
      <ShortName>Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureDebtTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>40602 - Disclosure - Debt - Outstanding borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtOutstandingBorrowingsDetails</Role>
      <ShortName>Debt - Outstanding borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>40603 - Disclosure - Debt - Maturities of debt obligations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtMaturitiesOfDebtObligationsDetails</Role>
      <ShortName>Debt - Maturities of debt obligations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>40604 - Disclosure - Debt - BCSF Revolving Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtBcsfRevolvingCreditFacilityDetails</Role>
      <ShortName>Debt - BCSF Revolving Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>40605 - Disclosure - Debt - Components of interest expense BCSF Revolving Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtComponentsOfInterestExpenseBcsfRevolvingCreditFacilityDetails</Role>
      <ShortName>Debt - Components of interest expense BCSF Revolving Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>40606 - Disclosure - Debt - 2018-1 Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebt20181NotesDetails</Role>
      <ShortName>Debt - 2018-1 Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>40607 - Disclosure - Debt - Interest expense on 2018-1 Notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtInterestExpenseOn20181NotesDetails</Role>
      <ShortName>Debt - Interest expense on 2018-1 Notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>40608 - Disclosure - Debt - JPM Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtJpmCreditFacilityDetails</Role>
      <ShortName>Debt - JPM Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>40609 - Disclosure - Debt - Interest expense JPM Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtInterestExpenseJpmCreditFacilityDetails</Role>
      <ShortName>Debt - Interest expense JPM Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>40610 - Disclosure - Debt - 2019 1 Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebt20191DebtDetails</Role>
      <ShortName>Debt - 2019 1 Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>40611 - Disclosure - Debt - Schedule of 2019 1 Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtScheduleOf20191DebtDetails</Role>
      <ShortName>Debt - Schedule of 2019 1 Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>40612 - Disclosure - Debt - Interest expense on 2019-1 Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtInterestExpenseOn20191DebtDetails</Role>
      <ShortName>Debt - Interest expense on 2019-1 Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>40613 - Disclosure - Debt - Revolving advisor loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtRevolvingAdvisorLoanDetails</Role>
      <ShortName>Debt - Revolving advisor loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>40614 - Disclosure - Debt - 2023 notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebt2023NotesDetails</Role>
      <ShortName>Debt - 2023 notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>40615 - Disclosure - Debt - 2023 notes Interest expense and carrying value (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebt2023NotesInterestExpenseAndCarryingValueDetails</Role>
      <ShortName>Debt - 2023 notes Interest expense and carrying value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>40616 - Disclosure - Debt - March 2026 notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtMarch2026NotesDetails</Role>
      <ShortName>Debt - March 2026 notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>40617 - Disclosure - Debt - March 2026 notes Interest expense and carrying value (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtMarch2026NotesInterestExpenseAndCarryingValueDetails</Role>
      <ShortName>Debt - March 2026 notes Interest expense and carrying value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>40618 - Disclosure - Debt - October 2026 notes (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtOctober2026NotesDetails</Role>
      <ShortName>Debt - October 2026 notes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>40619 - Disclosure - Debt - October 2026 notes Interest expense and carrying value (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtOctober2026NotesInterestExpenseAndCarryingValueDetails</Role>
      <ShortName>Debt - October 2026 notes Interest expense and carrying value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>40620 - Disclosure - Debt - Sumitomo Credit Facility (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtSumitomoCreditFacilityDetails</Role>
      <ShortName>Debt - Sumitomo Credit Facility (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>40621 - Disclosure - Debt - Sumitomo Credit Facility Interest expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDebtSumitomoCreditFacilityInterestExpenseDetails</Role>
      <ShortName>Debt - Sumitomo Credit Facility Interest expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDerivativesDetails</Role>
      <ShortName>Derivatives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureDerivativesTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>40702 - Disclosure - Derivatives - Offset of assets and liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDerivativesOffsetOfAssetsAndLiabilitiesDetails</Role>
      <ShortName>Derivatives - Offset of assets and liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>40703 - Disclosure - Derivatives - Effect of transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDerivativesEffectOfTransactionsDetails</Role>
      <ShortName>Derivatives - Effect of transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Distributions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureDistributionsDetails</Role>
      <ShortName>Distributions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureDistributionsTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Common Stock Capital (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureCommonStockCapitalDetails</Role>
      <ShortName>Common Stock Capital (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureCommonStockCapital</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Commitments and Contingencies - Summary of Unfunded Commitments under Loan and Financing Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureCommitmentsAndContingenciesSummaryOfUnfundedCommitmentsUnderLoanAndFinancingAgreementsDetails</Role>
      <ShortName>Commitments and Contingencies - Summary of Unfunded Commitments under Loan and Financing Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Financial Highlights (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://baincapital.com/role/DisclosureFinancialHighlightsDetails</Role>
      <ShortName>Financial Highlights (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://baincapital.com/role/DisclosureFinancialHighlightsTables</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="bcsf-20220930x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>995470 - Disclosure - N-2</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/cef/role/N2</Role>
      <ShortName>N-2</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Warning">[ix-0514-Hidden-Fact-Eligible-For-Transform] WARN: 2085 fact(s) appearing in ix:hidden were eligible for transformation: bcsf:IncentiveFeeLookbackTerm, bcsf:InvestmentOwnedMeasurementInput, bcsf:NetInvestmentIncomePerShareDiluted, bcsf:PeriodForRepaymentOfDebt, bcsf:PeriodForRepaymentOfLineOfCredit, us-gaap-supplement:InvestmentInterestRateFloor, us-gaap-supplement:InvestmentInterestRatePaidInKind, us-gaap-supplement:InvestmentNonIncomeProducing, us-gaap-supplement:InvestmentRestrictionStatus, us-gaap-supplement:InvestmentSignificantUnobservableInput, us-gaap:CommonStockSharesIssued, us-gaap:CommonStockSharesOutstanding, us-gaap:EarningsPerShareDiluted, us-gaap:InvestmentOwnedPercentOfNetAssets, us-gaap:LimitedLiabilityCompanyLLCOrLimitedPartnershipLPMembersOrLimitedPartnersOwnershipInterest, us-gaap:WeightedAverageNumberOfDilutedSharesOutstanding -  bcsf-20220930x10q.htm 9</Log>
    <Log type="Warning">[ix-0514-Hidden-Fact-Not-Referenced] WARN: 1274 fact(s) appearing in ix:hidden were not referenced by any -sec-ix-hidden style property: us-gaap-supplement:InvestmentIndustrySectorExtensibleEnumeration, us-gaap-supplement:InvestmentIssuerAffiliationExtensibleEnumeration, us-gaap-supplement:InvestmentVariableInterestRateTypeExtensibleEnumeration, us-gaap:CommitmentsAndContingencies, us-gaap:DerivativeAssetTypeExtensibleList -  bcsf-20220930x10q.htm 9</Log>
    <Log type="Warning">[dq-0712-Presentation-Base-Set-Order] Role '41201 - Disclosure - Subsequent Events (Details)', a level 4, Detail role, appears before '995470 - Disclosure - N-2', a level 1, Note role. bcsf-20220930.xsd 417, https://xbrl.sec.gov/cef/2022/cef-2022.xsd 29</Log>
  </Logs>
  <InputFiles>
    <File doctype="10-Q" original="bcsf-20220930x10q.htm">bcsf-20220930x10q.htm</File>
    <File>bcsf-20220930.xsd</File>
    <File>bcsf-20220930_cal.xml</File>
    <File>bcsf-20220930_def.xml</File>
    <File>bcsf-20220930_lab.xml</File>
    <File>bcsf-20220930_pre.xml</File>
    <File>bcsf-20220930xex10d26.htm</File>
    <File>bcsf-20220930xex10d28.htm</File>
    <File>bcsf-20220930xex31d1.htm</File>
    <File>bcsf-20220930xex31d2.htm</File>
    <File>bcsf-20220930xex32.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="6229">http://fasb.org/us-gaap-sup/2022q3</BaseTaxonomy>
    <BaseTaxonomy items="5417">http://fasb.org/us-gaap/2022</BaseTaxonomy>
    <BaseTaxonomy items="2">http://xbrl.sec.gov/cef/2022</BaseTaxonomy>
    <BaseTaxonomy items="30">http://xbrl.sec.gov/dei/2022</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
