<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.25.3</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>388</ContextCount>
  <ElementCount>299</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>67</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001 - Statement - Consolidated Statements of Financial Condition (Current Period Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-financial-condition-current-period-unaudited</Role>
      <ShortName>Consolidated Statements of Financial Condition (Current Period Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>002 - Statement - Consolidated Statements of Financial Condition (Current Period Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-financial-condition-current-period-unaudited-parentheticals</Role>
      <ShortName>Consolidated Statements of Financial Condition (Current Period Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>003 - Statement - Consolidated Statements of Operations (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-operations-unaudited</Role>
      <ShortName>Consolidated Statements of Operations (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>004 - Statement - Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-comprehensive-income-unaudited</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>005 - Statement - Consolidated Statements of Changes In Stockholders' Equity (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-changes-in-stockholders-equity-unaudited</Role>
      <ShortName>Consolidated Statements of Changes In Stockholders' Equity (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>006 - Statement - Consolidated Statements of Changes In Stockholders' Equity (Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-changes-in-stockholders-equity-unaudited-parentheticals</Role>
      <ShortName>Consolidated Statements of Changes In Stockholders' Equity (Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>007 - Statement - Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-consolidated-statements-of-cash-flows-unaudited</Role>
      <ShortName>Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 1 - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-1-summary-of-significant-accounting-policies</Role>
      <ShortName>Note 1 - Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 2 - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-2-earnings-per-share</Role>
      <ShortName>Note 2 - Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 3 - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-3-securities</Role>
      <ShortName>Note 3 - Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 4 - Loans Receivable</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable</Role>
      <ShortName>Note 4 - Loans Receivable</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 5 - Foreclosed Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-</Role>
      <ShortName>Note 5 - Foreclosed Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 6 - Borrowings and Subordinated Notes</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-6-borrowings-and-subordinated-notes</Role>
      <ShortName>Note 6 - Borrowings and Subordinated Notes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 7 - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value</Role>
      <ShortName>Note 7 - Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 8 - Revenue From Contracts With Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-8-revenue-from-contracts-with-customers</Role>
      <ShortName>Note 8 - Revenue From Contracts With Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.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>17</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995462 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-1-summary-of-significant-accounting-policies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995463 - Disclosure - Note 2 - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-2-earnings-per-share-tables</Role>
      <ShortName>Note 2 - Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-2-earnings-per-share</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995464 - Disclosure - Note 3 - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-3-securities-tables</Role>
      <ShortName>Note 3 - Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-3-securities</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Note 4 - Loans Receivable (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-tables</Role>
      <ShortName>Note 4 - Loans Receivable (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995466 - Disclosure - Note 5 - Foreclosed Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-tables</Role>
      <ShortName>Note 5 - Foreclosed Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995467 - Disclosure - Note 6 - Borrowings and Subordinated Notes (Tables)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-6-borrowings-and-subordinated-notes-tables</Role>
      <ShortName>Note 6 - Borrowings and Subordinated Notes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-6-borrowings-and-subordinated-notes</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995468 - Disclosure - Note 7 - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-tables</Role>
      <ShortName>Note 7 - Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995469 - Disclosure - Note 8 - Revenue From Contracts With Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-8-revenue-from-contracts-with-customers-tables</Role>
      <ShortName>Note 8 - Revenue From Contracts With Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-8-revenue-from-contracts-with-customers</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995470 - Disclosure - Note 1 - Summary of Significant Accounting Policies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-1-summary-of-significant-accounting-policies-details-textual</Role>
      <ShortName>Note 1 - Summary of Significant Accounting Policies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995471 - Disclosure - Note 2 - Earnings Per Share - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-2-earnings-per-share-earnings-per-share-details</Role>
      <ShortName>Note 2 - Earnings Per Share - Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995472 - Disclosure - Note 3 - Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-3-securities-details-textual</Role>
      <ShortName>Note 3 - Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-3-securities-tables</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995473 - Disclosure - Note 3 - Securities - Fair Value of Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-3-securities-fair-value-of-securities-details</Role>
      <ShortName>Note 3 - Securities - Fair Value of Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995474 - Disclosure - Note 3 - Securities - Amortized Cost and Fair Value of Securities By Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-3-securities-amortized-cost-and-fair-value-of-securities-by-contractual-maturity-details</Role>
      <ShortName>Note 3 - Securities - Amortized Cost and Fair Value of Securities By Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Note 3 - Securities - Securities With Unrealized Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-3-securities-securities-with-unrealized-losses-details</Role>
      <ShortName>Note 3 - Securities - Securities With Unrealized Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995476 - Disclosure - Note 4 - Loans Receivable (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-details-textual</Role>
      <ShortName>Note 4 - Loans Receivable (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-tables</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995477 - Disclosure - Note 4 - Loans Receivable - Loans Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-loans-receivable-details</Role>
      <ShortName>Note 4 - Loans Receivable - Loans Receivable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995478 - Disclosure - Note 4 - Loans Receivable - Activity in the Allowance for Loan Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-activity-in-the-allowance-for-loan-losses-details</Role>
      <ShortName>Note 4 - Loans Receivable - Activity in the Allowance for Loan Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995479 - Disclosure - Note 4 - Loans Receivable - Allowance for Loan Losses and Loans Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-allowance-for-loan-losses-and-loans-receivable-details</Role>
      <ShortName>Note 4 - Loans Receivable - Allowance for Loan Losses and Loans Receivable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995480 - Disclosure - Note 4 - Loans Receivable - Loans Individually Evaluated for Impairment By Class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-loans-individually-evaluated-for-impairment-by-class-details</Role>
      <ShortName>Note 4 - Loans Receivable - Loans Individually Evaluated for Impairment By Class (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995481 - Disclosure - Note 4 - Loans Receivable - Recorded Investment in Nonaccrual Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-recorded-investment-in-nonaccrual-loans-details</Role>
      <ShortName>Note 4 - Loans Receivable - Recorded Investment in Nonaccrual Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995482 - Disclosure - Note 4 - Loans Receivable - Aging of the Recorded Investment of Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-aging-of-the-recorded-investment-of-loans-details</Role>
      <ShortName>Note 4 - Loans Receivable - Aging of the Recorded Investment of Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995483 - Disclosure - Note 4 - Loans Receivable - Risk Categories of Loans By Class of Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-4-loans-receivable-risk-categories-of-loans-by-class-of-loans-details</Role>
      <ShortName>Note 4 - Loans Receivable - Risk Categories of Loans By Class of Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995484 - Disclosure - Note 5 - Foreclosed Assets (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-details-textual</Role>
      <ShortName>Note 5 - Foreclosed Assets (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-tables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Note 5 - Foreclosed Assets - Real Estate Acquired Through Foreclosure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-real-estate-acquired-through-foreclosure-details</Role>
      <ShortName>Note 5 - Foreclosed Assets - Real Estate Acquired Through Foreclosure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995486 - Disclosure - Note 5 - Foreclosed Assets- Roll Forward of Foreclosed Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-roll-forward-of-foreclosed-assets-details</Role>
      <ShortName>Note 5 - Foreclosed Assets- Roll Forward of Foreclosed Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995487 - Disclosure - Note 5 - Foreclosed Assets - Activity in the Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-5-foreclosed-assets-activity-in-the-valuation-allowance-details</Role>
      <ShortName>Note 5 - Foreclosed Assets - Activity in the Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995488 - Disclosure - Note 6 - Borrowings and Subordinated Notes (Details Textual)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-6-borrowings-and-subordinated-notes-details-textual</Role>
      <ShortName>Note 6 - Borrowings and Subordinated Notes (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-6-borrowings-and-subordinated-notes-tables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995489 - Disclosure - Note 6 - Borrowings and Subordinated Notes - Advances From the FHLB (Details)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-6-borrowings-and-subordinated-notes-advances-from-the-fhlb-details</Role>
      <ShortName>Note 6 - Borrowings and Subordinated Notes - Advances From the FHLB (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995490 - Disclosure - Note 7 - Fair Value (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-details-textual</Role>
      <ShortName>Note 7 - Fair Value (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-tables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995491 - Disclosure - Note 7 - Fair Value - Financial Assets At Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-financial-assets-at-fair-value-details</Role>
      <ShortName>Note 7 - Fair Value - Financial Assets At Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995492 - Disclosure - Note 7 - Fair Value - Financial Assets At Fair Value on Non-recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-financial-assets-at-fair-value-on-nonrecurring-basis-details</Role>
      <ShortName>Note 7 - Fair Value - Financial Assets At Fair Value on Non-recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995493 - Disclosure - Note 7 - Fair Value - Quantitative Information for Financial Instruments on Non-recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-quantitative-information-for-financial-instruments-on-nonrecurring-basis-details</Role>
      <ShortName>Note 7 - Fair Value - Quantitative Information for Financial Instruments on Non-recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995494 - Disclosure - Note 7 - Fair Value - Carrying Amount and Estimated Fair Value of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-7-fair-value-carrying-amount-and-estimated-fair-value-of-financial-instruments-details</Role>
      <ShortName>Note 7 - Fair Value - Carrying Amount and Estimated Fair Value of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Note 8 - Revenue From Contracts With Customers (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-8-revenue-from-contracts-with-customers-details-textual</Role>
      <ShortName>Note 8 - Revenue From Contracts With Customers (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20250930/role/statement-note-8-revenue-from-contracts-with-customers-tables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="bfin20250930_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995496 - Disclosure - Note 8 - Revenue From Contracts With Customers - Noninterest Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20250930/role/statement-note-8-revenue-from-contracts-with-customers-noninterest-income-details</Role>
      <ShortName>Note 8 - Revenue From Contracts With Customers - Noninterest Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>bfin-20250930.xsd</File>
    <File>bfin-20250930_cal.xml</File>
    <File>bfin-20250930_def.xml</File>
    <File>bfin-20250930_lab.xml</File>
    <File>bfin-20250930_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="bfin20250930_10q.htm">bfin20250930_10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1680">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="5">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
