<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>544</ContextCount>
  <ElementCount>462</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>83</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>75000 - Document - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - Consolidated Balance Sheets (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/StatementConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Consolidated Balance Sheets (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75020 - Statement - Consolidated Balance Sheets (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/StatementConsolidatedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75030 - Statement - Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ConsolidatedStatementsofIncomeUnaudited</Role>
      <ShortName>Consolidated Statements of Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75040 - Statement - Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ConsolidatedStatementsofComprehensiveIncomeUnaudited</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>75050 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ConsolidatedStatementsofChangesinStockholdersEquityUnaudited</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>75060 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ConsolidatedStatementsofChangesinStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>75070 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ConsolidatedStatementsofCashFlowsUnaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Nature of Operations and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/NatureofOperationsandSummaryofSignificantAccountingPolicies</Role>
      <ShortName>Nature of Operations and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Earnings Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/EarningsPerCommonShare</Role>
      <ShortName>Earnings Per Common Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Share-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ShareBasedCompensation</Role>
      <ShortName>Share-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/Securities</Role>
      <ShortName>Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/LoansLeaseReceivablesandAllowanceforCreditLosses</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/Leases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Other Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/OtherAssets</Role>
      <ShortName>Other Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/Deposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995535 - Disclosure - FHLB Advances, Other Borrowings and Subordinated Notes and Debentures</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/FHLBAdvancesOtherBorrowingsandSubordinatedNotesandDebentures</Role>
      <ShortName>FHLB Advances, Other Borrowings and Subordinated Notes and Debentures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Preferred Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosurePreferredStock</Role>
      <ShortName>Preferred Stock</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/CommitmentsandContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Fair Value Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/FairValueDisclosures</Role>
      <ShortName>Fair Value Disclosures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DerivativeFinancialInstruments</Role>
      <ShortName>Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995585 - Disclosure - Regulatory Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/RegulatoryCapital</Role>
      <ShortName>Regulatory Capital</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995595 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995605 - Disclosure - Nature of Operations and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Nature of Operations and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/NatureofOperationsandSummaryofSignificantAccountingPolicies</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Earnings Per Common Share (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/EarningsPerCommonSharePolicies</Role>
      <ShortName>Earnings Per Common Share (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/NatureofOperationsandSummaryofSignificantAccountingPolicies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995625 - Disclosure - Share-Based Compensation (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ShareBasedCompensationPolicies</Role>
      <ShortName>Share-Based Compensation (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/NatureofOperationsandSummaryofSignificantAccountingPolicies</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Derivative Financial Instruments (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DerivativeFinancialInstrumentsPolicies</Role>
      <ShortName>Derivative Financial Instruments (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/NatureofOperationsandSummaryofSignificantAccountingPolicies</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Earnings Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/EarningsPerCommonShareTables</Role>
      <ShortName>Earnings Per Common Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/EarningsPerCommonShare</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Share-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/ShareBasedCompensationTables</Role>
      <ShortName>Share-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/ShareBasedCompensation</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995665 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/SecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/Securities</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/LoansLeaseReceivablesandAllowanceforCreditLossesTables</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/LoansLeaseReceivablesandAllowanceforCreditLosses</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995685 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/LeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/Leases</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Other Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/OtherAssetsTables</Role>
      <ShortName>Other Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/OtherAssets</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/Deposits</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995715 - Disclosure - FHLB Advances, Other Borrowings and Subordinated Notes and Debentures (Tables)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/FHLBAdvancesOtherBorrowingsandSubordinatedNotesandDebenturesTables</Role>
      <ShortName>FHLB Advances, Other Borrowings and Subordinated Notes and Debentures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/FHLBAdvancesOtherBorrowingsandSubordinatedNotesandDebentures</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/CommitmentsandContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/CommitmentsandContingencies</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995735 - Disclosure - Fair Value Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/FairValueDisclosuresTables</Role>
      <ShortName>Fair Value Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/FairValueDisclosures</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Derivative Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DerivativeFinancialInstrumentsTables</Role>
      <ShortName>Derivative Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/DerivativeFinancialInstruments</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Regulatory Capital (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/RegulatoryCapitalTables</Role>
      <ShortName>Regulatory Capital (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/RegulatoryCapital</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSegmentInformation</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Earnings Per Common Share - Summary of Earnings Per Common Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureEarningsPerCommonShareSummaryOfEarningsPerCommonShareDetails</Role>
      <ShortName>Earnings Per Common Share - Summary of Earnings Per Common Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995785 - Disclosure - Share-Based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSharebasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Share-Based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Share-Based Compensation - Summary of Restricted Stock Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSharebasedCompensationSummaryOfRestrictedStockActivityDetails</Role>
      <ShortName>Share-Based Compensation - Summary of Restricted Stock Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995805 - Disclosure - Share-Based Compensation - Summary of Share-Based Compensation Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSharebasedCompensationSummaryOfSharebasedCompensationExpenseDetails</Role>
      <ShortName>Share-Based Compensation - Summary of Share-Based Compensation Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Securities - Schedule of Available-for-sale Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSecuritiesScheduleOfAvailableforsaleSecuritiesDetails</Role>
      <ShortName>Securities - Schedule of Available-for-sale Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995825 - Disclosure - Securities - Schedule of Held-to-maturity Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSecuritiesScheduleOfHeldtomaturitySecuritiesDetails</Role>
      <ShortName>Securities - Schedule of Held-to-maturity Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Securities - Summary of Amortized Cost and Fair Value by Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSecuritiesSummaryOfAmortizedCostAndFairValueByContractualMaturityDetails</Role>
      <ShortName>Securities - Summary of Amortized Cost and Fair Value by Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Securities - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSecuritiesAdditionalInformationDetails</Role>
      <ShortName>Securities - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Securities - Summary of Unrealized Loss for Securities Available-For-Sale (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSecuritiesSummaryOfUnrealizedLossForSecuritiesAvailableforsaleDetails</Role>
      <ShortName>Securities - Summary of Unrealized Loss for Securities Available-For-Sale (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Securities - Summary of Unrecognized Loss for Securities Held-To-Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSecuritiesSummaryOfUnrecognizedLossForSecuritiesHeldtomaturityDetails</Role>
      <ShortName>Securities - Summary of Unrecognized Loss for Securities Held-To-Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Loan and Lease Receivables Composition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesLoanAndLeaseReceivablesCompositionDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Loan and Lease Receivables Composition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Loans Transferred and Purchased (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesLoansTransferredAndPurchasedDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Loans Transferred and Purchased (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Financing Receivable by Credit Quality Indicators (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesFinancingReceivableByCreditQualityIndicatorsDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Financing Receivable by Credit Quality Indicators (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses -Delinquency Aging of Loan and Lease Portfolio by Class of Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesDelinquencyAgingOfLoanAndLeasePortfolioByClassOfReceivableDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses -Delinquency Aging of Loan and Lease Portfolio by Class of Receivable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995915 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Non-Accrual Loans and Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesScheduleOfNonaccrualLoansAndLeasesDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Non-Accrual Loans and Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995925 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Collateral Dependent commercial and industrial loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesScheduleOfCollateralDependentCommercialAndIndustrialLoansDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Collateral Dependent commercial and industrial loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995935 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Amortized Cost Basis of Loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesScheduleOfAmortizedCostBasisOfLoanDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Amortized Cost Basis of Loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995945 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Corporation Closely Monitors the Performance of Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesScheduleOfCorporationCloselyMonitorsThePerformanceOfLoansDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Corporation Closely Monitors the Performance of Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995955 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Financial Effect of Loan Modifications Presented to Borrowers Experiencing Financial Difficulty (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesScheduleOfFinancialEffectOfLoanModificationsPresentedToBorrowersExperiencingFina</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Financial Effect of Loan Modifications Presented to Borrowers Experiencing Financial Difficulty (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>995965 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Amortized Cost Basis of Loans That Had Payment Default (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesScheduleOfAmortizedCostBasisOfLoansThatHadPaymentDefaultDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Schedule of Amortized Cost Basis of Loans That Had Payment Default (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>995975 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Summary of Activity in Allowance for Credit Losses by Portfolio Segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesSummaryOfActivityInAllowanceForCreditLossesByPortfolioSegmentDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Summary of Activity in Allowance for Credit Losses by Portfolio Segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>995985 - Disclosure - Loans, Lease Receivables, and Allowance for Credit Losses - Allowance for Credit Losses and Balances by Type of Allowance Methodology (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLoansLeaseReceivablesAndAllowanceForCreditLossesAllowanceForCreditLossesAndBalancesByTypeOfAllowanceMethodologyDetails</Role>
      <ShortName>Loans, Lease Receivables, and Allowance for Credit Losses - Allowance for Credit Losses and Balances by Type of Allowance Methodology (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>996005 - Disclosure - Leases - Components of Lease expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLeasesComponentsOfLeaseExpenseDetails</Role>
      <ShortName>Leases - Components of Lease expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>996015 - Disclosure - Leases - Operating Lease Quantitative Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLeasesOperatingLeaseQuantitativeInformationDetails</Role>
      <ShortName>Leases - Operating Lease Quantitative Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>996025 - Disclosure - Leases - Maturity of Operating Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureLeasesMaturityOfOperatingLeaseLiabilitiesDetails</Role>
      <ShortName>Leases - Maturity of Operating Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>996045 - Disclosure - Other Assets - Summary of Investment In and Unfunded Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureOtherAssetsSummaryOfInvestmentInAndUnfundedCommitmentsDetails</Role>
      <ShortName>Other Assets - Summary of Investment In and Unfunded Commitments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>996055 - Disclosure - Other Assets - Summary of Accrued Interest Receivable and Other Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureOtherAssetsSummaryOfAccruedInterestReceivableAndOtherAssetsDetails</Role>
      <ShortName>Other Assets - Summary of Accrued Interest Receivable and Other Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>996065 - Disclosure - Other Assets - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureOtherAssetsAdditionalInformationDetails</Role>
      <ShortName>Other Assets - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>996075 - Disclosure - Deposits - Composition Of Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureDepositsCompositionOfDepositsDetails</Role>
      <ShortName>Deposits - Composition Of Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>996085 - Disclosure - Deposits - Time Deposits by Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureDepositsTimeDepositsByMaturityDetails</Role>
      <ShortName>Deposits - Time Deposits by Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>996095 - Disclosure - Deposits - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureDepositsAdditionalInformationDetails</Role>
      <ShortName>Deposits - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>996105 - Disclosure - FHLB Advances, Other Borrowings and Subordinated Notes and Debentures - Composition of Borrowed Funds (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFhlbAdvancesOtherBorrowingsAndSubordinatedNotesAndDebenturesCompositionOfBorrowedFundsDetails</Role>
      <ShortName>FHLB Advances, Other Borrowings and Subordinated Notes and Debentures - Composition of Borrowed Funds (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>996115 - Disclosure - FHLB Advances, Other Borrowings and Subordinated Notes and Debentures - Summary of Annual Maturities of Borrowings (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFhlbAdvancesOtherBorrowingsAndSubordinatedNotesAndDebenturesSummaryOfAnnualMaturitiesOfBorrowingsDetails</Role>
      <ShortName>FHLB Advances, Other Borrowings and Subordinated Notes and Debentures - Summary of Annual Maturities of Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>996125 - Disclosure - FHLB Advances, Other Borrowings and Subordinated Notes and Debentures - Additional Information (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFhlbAdvancesOtherBorrowingsAndSubordinatedNotesAndDebenturesAdditionalInformationDetails</Role>
      <ShortName>FHLB Advances, Other Borrowings and Subordinated Notes and Debentures - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>996135 - Disclosure - Preferred Stock - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosurePreferredStockAdditionalInformationDetails</Role>
      <ShortName>Preferred Stock - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>996145 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>996155 - Disclosure - Commitments and Contingencies - SBA Recourse Reserve (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureCommitmentsAndContingenciesSbaRecourseReserveDetails</Role>
      <ShortName>Commitments and Contingencies - SBA Recourse Reserve (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>996165 - Disclosure - Fair Value Disclosures - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFairValueDisclosuresAdditionalInformationDetails</Role>
      <ShortName>Fair Value Disclosures - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>996175 - Disclosure - Fair Value Disclosures - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFairValueDisclosuresAssetsAndLiabilitiesMeasuredAtFairValueOnARecurringBasisDetails</Role>
      <ShortName>Fair Value Disclosures - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>996185 - Disclosure - Fair Value Disclosures - Assets and Liabilities Measured at Fair Value on a Non-recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFairValueDisclosuresAssetsAndLiabilitiesMeasuredAtFairValueOnANonrecurringBasisDetails</Role>
      <ShortName>Fair Value Disclosures - Assets and Liabilities Measured at Fair Value on a Non-recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>996195 - Disclosure - Fair Value Disclosures - Estimated Fair Values For Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureFairValueDisclosuresEstimatedFairValuesForFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Disclosures - Estimated Fair Values For Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>996205 - Disclosure - Derivative Financial Instruments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureDerivativeFinancialInstrumentsAdditionalInformationDetails</Role>
      <ShortName>Derivative Financial Instruments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>996215 - Disclosure - Derivative Financial Instruments - Summary of Location and Fair Value of Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureDerivativeFinancialInstrumentsSummaryOfLocationAndFairValueOfDerivativeInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments - Summary of Location and Fair Value of Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>996225 - Disclosure - Regulatory Capital - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureRegulatoryCapitalAdditionalInformationDetails</Role>
      <ShortName>Regulatory Capital - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>996235 - Disclosure - Regulatory Capital - Regulatory Capital Ratios (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureRegulatoryCapitalRegulatoryCapitalRatiosDetails</Role>
      <ShortName>Regulatory Capital - Regulatory Capital Ratios (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>996245 - Disclosure - Regulatory Capital - Regulatory Capital Ratios (Parenthetical) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureRegulatoryCapitalRegulatoryCapitalRatiosParentheticalDetails</Role>
      <ShortName>Regulatory Capital - Regulatory Capital Ratios (Parenthetical) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="fbiz-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>996255 - Disclosure - Segment Information - Schedule of Segment Reporting Information, by Segments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://firstbusiness.bank/20260331/taxonomy/role/DisclosureSegmentInformationScheduleOfSegmentReportingInformationBySegmentsDetails</Role>
      <ShortName>Segment Information - Schedule of Segment Reporting Information, by Segments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="fbiz-20260331.htm">fbiz-20260331.htm</File>
    <File>fbiz-20260331.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="2248">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>