<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>418</ContextCount>
  <ElementCount>346</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>63</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="bfin20200423_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/20200630/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="bfin20200423_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/20200630/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="bfin20200423_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/20200630/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="bfin20200423_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/20200630/role/statement-consolidated-statements-of-operations-unaudited</Role>
      <ShortName>Consolidated Statements of Operations (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="bfin20200423_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/20200630/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="bfin20200423_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/20200630/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="bfin20200423_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/20200630/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="bfin20200423_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/20200630/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="bfin20200423_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/20200630/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="bfin20200423_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/20200630/role/statement-note-2-earnings-per-share</Role>
      <ShortName>Note 2 - Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="bfin20200423_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/20200630/role/statement-note-3-securities</Role>
      <ShortName>Note 3 - Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="bfin20200423_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/20200630/role/statement-note-4-loans-receivable</Role>
      <ShortName>Note 4 - Loans Receivable</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 5 - Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned</Role>
      <ShortName>Note 5 - Other Real Estate Owned</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 6 - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-6-leases</Role>
      <ShortName>Note 6 - Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 7 - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings</Role>
      <ShortName>Note 7 - Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 8 - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value</Role>
      <ShortName>Note 8 - Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 9 - Revenue from Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-9-revenue-from-contracts-with-customers</Role>
      <ShortName>Note 9 - Revenue from Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-1-summary-of-significant-accounting-policies</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 2 - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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/20200630/role/statement-note-2-earnings-per-share</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 3 - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-3-securities-tables</Role>
      <ShortName>Note 3 - Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-3-securities</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 4 - Loans Receivable (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-4-loans-receivable-tables</Role>
      <ShortName>Note 4 - Loans Receivable (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-4-loans-receivable</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 5 - Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned-tables</Role>
      <ShortName>Note 5 - Other Real Estate Owned (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 6 - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-6-leases-tables</Role>
      <ShortName>Note 6 - Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-6-leases</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>023 - Disclosure - Note 7 - Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings-tables</Role>
      <ShortName>Note 7 - Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>024 - Disclosure - Note 8 - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value-tables</Role>
      <ShortName>Note 8 - Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>025 - Disclosure - Note 9 - Revenue from Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-9-revenue-from-contracts-with-customers-tables</Role>
      <ShortName>Note 9 - Revenue from Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-9-revenue-from-contracts-with-customers</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>026 - Disclosure - Note 2 - Earnings Per Share - Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>027 - Disclosure - Note 3 - Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-3-securities-details-textual</Role>
      <ShortName>Note 3 - Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-3-securities-tables</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>028 - Disclosure - Note 3 - Securities - Fair Value of Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>029 - Disclosure - Note 3 - Securities - Amortized Cost and Fair Value of Securities By Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>030 - Disclosure - Note 3 - Securities - Sales of Equity Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-3-securities-sales-of-equity-securities-details</Role>
      <ShortName>Note 3 - Securities - Sales of Equity Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>031 - Disclosure - Note 3 - Securities - Securities with Unrealized Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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>32</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>032 - Disclosure - Note 4 - Loans Receivable (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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/20200630/role/statement-note-4-loans-receivable-tables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>033 - Disclosure - Note 4 - Loans Receivable - Loans Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-4-loans-receivable-loans-receivable-details</Role>
      <ShortName>Note 4 - Loans Receivable - Loans Receivable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>034 - Disclosure - Note 4 - Loans Receivable - Allowance for Loan Losses and Loans Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>035 - Disclosure - Note 4 - Loans Receivable - Activity in the Allowance for Loan Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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>36</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>036 - Disclosure - Note 4 - Loans Receivable - Loans Individually Evaluated for Impairment By Class (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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>37</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>037 - Disclosure - Note 4 - Loans Receivable - Recorded Investment in Nonaccrual Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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>38</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>038 - Disclosure - Note 4 - Loans Receivable - Aging of the Recorded Investment of Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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>39</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>039 - Disclosure - Note 4 - Loans Receivable - PPP activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-4-loans-receivable-ppp-activity-details</Role>
      <ShortName>Note 4 - Loans Receivable - PPP activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>040 - Disclosure - Note 4 - Loans Receivable - COVID Loan Forbearance Program (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-4-loans-receivable-covid-loan-forbearance-program-details</Role>
      <ShortName>Note 4 - Loans Receivable - COVID Loan Forbearance Program (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>041 - Disclosure - Note 4 - Loans Receivable - Risk Categories of Loans By Class of Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/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>42</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>042 - Disclosure - Note 5 - Other Real Estate Owned (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned-details-textual</Role>
      <ShortName>Note 5 - Other Real Estate Owned (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned-tables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>043 - Disclosure - Note 5 - Other Real Estate Owned - Real Estate Acquired Through Foreclosure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned-real-estate-acquired-through-foreclosure-details</Role>
      <ShortName>Note 5 - Other Real Estate Owned - Real Estate Acquired Through Foreclosure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>044 - Disclosure - Note 5 - Other Real Estate Owned - Roll Forward of OREO (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned-roll-forward-of-oreo-details</Role>
      <ShortName>Note 5 - Other Real Estate Owned - Roll Forward of OREO (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>045 - Disclosure - Note 5 - Other Real Estate Owned - Activity in the Valuation Allowance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-5-other-real-estate-owned-activity-in-the-valuation-allowance-details</Role>
      <ShortName>Note 5 - Other Real Estate Owned - Activity in the Valuation Allowance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>046 - Disclosure - Note 6 - Leases (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-6-leases-details-textual</Role>
      <ShortName>Note 6 - Leases (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-6-leases-tables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>047 - Disclosure - Note 6 - Leases - Classification of Right of Use and Lease Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-6-leases-classification-of-right-of-use-and-lease-liabilities-details</Role>
      <ShortName>Note 6 - Leases - Classification of Right of Use and Lease Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>048 - Disclosure - Note 6 - Leases - Lease Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-6-leases-lease-cost-details</Role>
      <ShortName>Note 6 - Leases - Lease Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>049 - Disclosure - Note 6 - Leases - Future Minimum Payments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-6-leases-future-minimum-payments-details</Role>
      <ShortName>Note 6 - Leases - Future Minimum Payments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>050 - Disclosure - Note 7 - Borrowings (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings-details-textual</Role>
      <ShortName>Note 7 - Borrowings (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings-tables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>051 - Disclosure - Note 7 - Borrowings - Advances from the FHLB (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings-advances-from-the-fhlb-details</Role>
      <ShortName>Note 7 - Borrowings - Advances from the FHLB (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>052 - Disclosure - Note 7 - Borrowings - Securities Sold Under Agreements to Repurchase (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-7-borrowings-securities-sold-under-agreements-to-repurchase-details</Role>
      <ShortName>Note 7 - Borrowings - Securities Sold Under Agreements to Repurchase (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>053 - Disclosure - Note 8 - Fair Value (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value-details-textual</Role>
      <ShortName>Note 8 - Fair Value (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value-tables</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>054 - Disclosure - Note 8 - Fair Value - Financial Assets At Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value-financial-assets-at-fair-value-details</Role>
      <ShortName>Note 8 - Fair Value - Financial Assets At Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>055 - Disclosure - Note 8 - Fair Value - Carrying Amount and Estimated Fair Value of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-8-fair-value-carrying-amount-and-estimated-fair-value-of-financial-instruments-details</Role>
      <ShortName>Note 8 - Fair Value - Carrying Amount and Estimated Fair Value of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>056 - Disclosure - Note 9 - Revenue from Contracts with Customers (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-9-revenue-from-contracts-with-customers-details-textual</Role>
      <ShortName>Note 9 - Revenue from Contracts with Customers (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankfinancial.com/20200630/role/statement-note-9-revenue-from-contracts-with-customers-tables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="bfin20200423_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>057 - Disclosure - Note 9 - Revenues from Contracts with Customers - Noninterest Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankfinancial.com/20200630/role/statement-note-9-revenues-from-contracts-with-customers-noninterest-income-details</Role>
      <ShortName>Note 9 - Revenues from Contracts with Customers - Noninterest Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="bfin20200423_10q.htm">bfin20200423_10q.htm</File>
    <File>bfin-20200630.xsd</File>
    <File>bfin-20200630_cal.xml</File>
    <File>bfin-20200630_def.xml</File>
    <File>bfin-20200630_lab.xml</File>
    <File>bfin-20200630_pre.xml</File>
    <File>ex_186560.htm</File>
    <File>ex_186561.htm</File>
    <File>ex_186563.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/stpr/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
