<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>815</ContextCount>
  <ElementCount>522</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>128</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="fsbw20260630_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://investorrelations.fsbwa.com/20260630/role/statement-document-and-entity-information</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>001 - Statement - Consolidated Balance Sheets (Current Period Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-consolidated-balance-sheets-current-period-unaudited</Role>
      <ShortName>Consolidated Balance Sheets (Current Period Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>002 - Statement - Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-consolidated-balance-sheets-current-period-unaudited-parentheticals</Role>
      <ShortName>Consolidated Balance Sheets (Current Period Unaudited) (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>003 - Statement - Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-consolidated-statements-of-income-unaudited</Role>
      <ShortName>Consolidated Statements of Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fsbw20260630_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://investorrelations.fsbwa.com/20260630/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="fsbw20260630_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://investorrelations.fsbwa.com/20260630/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="fsbw20260630_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://investorrelations.fsbwa.com/20260630/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="fsbw20260630_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://investorrelations.fsbwa.com/20260630/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="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>008 - Disclosure - Note 1 - Basis of Presentation and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-1-basis-of-presentation-and-summary-of-significant-accounting-policies</Role>
      <ShortName>Note 1 - Basis of Presentation and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>009 - Disclosure - Note 2 - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments</Role>
      <ShortName>Note 2 - Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>010 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>011 - Disclosure - Note 4 - Mortgage Servicing Rights</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights</Role>
      <ShortName>Note 4 - Mortgage Servicing Rights</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>012 - Disclosure - Note 5 - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives</Role>
      <ShortName>Note 5 - Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>013 - Disclosure - Note 6 - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases</Role>
      <ShortName>Note 6 - Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>014 - Disclosure - Note 7 - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits</Role>
      <ShortName>Note 7 - Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>015 - Disclosure - Note 8 - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-8-commitments-and-contingencies</Role>
      <ShortName>Note 8 - Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>016 - Disclosure - Note 9 - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements</Role>
      <ShortName>Note 9 - Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>017 - Disclosure - Note 10 - Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-10-earnings-per-share</Role>
      <ShortName>Note 10 - Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>018 - Disclosure - Note 11 - Stock-based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation</Role>
      <ShortName>Note 11 - Stock-based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>019 - Disclosure - Note 12 - Regulatory Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-12-regulatory-capital</Role>
      <ShortName>Note 12 - Regulatory Capital</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>020 - Disclosure - Note 13 - Business Segments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-13-business-segments-</Role>
      <ShortName>Note 13 - Business Segments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>021 - Disclosure - Note 14 - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets</Role>
      <ShortName>Note 14 - Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>022 - Disclosure - Note 15 - Definitive Agreement</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-15-definitive-agreement</Role>
      <ShortName>Note 15 - Definitive Agreement</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.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>24</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995469 - Disclosure - Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-significant-accounting-policies-policies</Role>
      <ShortName>Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-1-basis-of-presentation-and-summary-of-significant-accounting-policies</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995470 - Disclosure - Note 2 - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-tables</Role>
      <ShortName>Note 2 - Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995471 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-tables</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995472 - Disclosure - Note 4 - Mortgage Servicing Rights (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights-tables</Role>
      <ShortName>Note 4 - Mortgage Servicing Rights (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995473 - Disclosure - Note 5 - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-tables</Role>
      <ShortName>Note 5 - Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995474 - Disclosure - Note 6 - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases-tables</Role>
      <ShortName>Note 6 - Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Note 7 - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits-tables</Role>
      <ShortName>Note 7 - Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995476 - Disclosure - Note 8 - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-8-commitments-and-contingencies-tables</Role>
      <ShortName>Note 8 - Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-8-commitments-and-contingencies</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995477 - Disclosure - Note 9 - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-tables</Role>
      <ShortName>Note 9 - Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995478 - Disclosure - Note 10 - Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-10-earnings-per-share-tables</Role>
      <ShortName>Note 10 - Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-10-earnings-per-share</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995479 - Disclosure - Note 11 - Stock-based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation-tables</Role>
      <ShortName>Note 11 - Stock-based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995480 - Disclosure - Note 12 - Regulatory Capital (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-12-regulatory-capital-tables</Role>
      <ShortName>Note 12 - Regulatory Capital (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-12-regulatory-capital</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995481 - Disclosure - Note 13 - Business Segments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-13-business-segments-tables</Role>
      <ShortName>Note 13 - Business Segments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-13-business-segments-</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995482 - Disclosure - Note 14 - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets-tables</Role>
      <ShortName>Note 14 - Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995483 - Disclosure - Note 1 - Basis of Presentation and Summary of Significant Accounting Policies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-1-basis-of-presentation-and-summary-of-significant-accounting-policies-details-textual</Role>
      <ShortName>Note 1 - Basis of Presentation and Summary of Significant Accounting Policies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995484 - Disclosure - Note 2 - Investments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-details-textual</Role>
      <ShortName>Note 2 - Investments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-tables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Note 2 - Investments - Schedule of Amortized Costs and Fair Values of Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-schedule-of-amortized-costs-and-fair-values-of-securities-details</Role>
      <ShortName>Note 2 - Investments - Schedule of Amortized Costs and Fair Values of Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995486 - Disclosure - Note 2 - Investments - Schedule of Debt Securities, Held-to-Maturity, Allowance for Credit Loss Roll Forward (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-schedule-of-debt-securities-heldtomaturity-allowance-for-credit-loss-roll-forward-details</Role>
      <ShortName>Note 2 - Investments - Schedule of Debt Securities, Held-to-Maturity, Allowance for Credit Loss Roll Forward (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995487 - Disclosure - Note 2 - Investments - Schedule of Debt Securities, Held-to-Maturity Credit Quality Indicators (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-schedule-of-debt-securities-heldtomaturity-credit-quality-indicators-details</Role>
      <ShortName>Note 2 - Investments - Schedule of Debt Securities, Held-to-Maturity Credit Quality Indicators (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995488 - Disclosure - Note 2 - Investments - Schedule of Securities Pledged as Collateral (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-schedule-of-securities-pledged-as-collateral-details</Role>
      <ShortName>Note 2 - Investments - Schedule of Securities Pledged as Collateral (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995489 - Disclosure - Note 2 - Investments - Schedule of Investments in Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-schedule-of-investments-in-unrealized-loss-position-details</Role>
      <ShortName>Note 2 - Investments - Schedule of Investments in Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995490 - Disclosure - Note 2 - Investments - Schedule of Contractual Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-2-investments-schedule-of-contractual-maturities-details</Role>
      <ShortName>Note 2 - Investments - Schedule of Contractual Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995491 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-details-textual</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-tables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995492 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Portfolio Loan Composition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-portfolio-loan-composition-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Portfolio Loan Composition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995493 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Allowance for Credit Losses by Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-allowance-for-credit-losses-by-category-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Allowance for Credit Losses by Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995494 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Modified Financing Receivables (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-modified-financing-receivables-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Modified Financing Receivables (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Financing Receivable, Modified, Past Due (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-financing-receivable-modified-past-due-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Financing Receivable, Modified, Past Due (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995496 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Past Due Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-past-due-loans-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Past Due Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995497 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Risk Rated Loan by Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-risk-rated-loan-by-category-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Risk Rated Loan by Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995498 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Amortized Cost of Nonaccrual Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-amortized-cost-of-nonaccrual-loans-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Amortized Cost of Nonaccrual Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995499 - Disclosure - Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Collateral Dependent Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-3-loans-receivable-and-allowance-for-credit-losses-loans-schedule-of-collateral-dependent-loans-details</Role>
      <ShortName>Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Collateral Dependent Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995500 - Disclosure - Note 4 - Mortgage Servicing Rights (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights-details-textual</Role>
      <ShortName>Note 4 - Mortgage Servicing Rights (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights-tables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>995501 - Disclosure - Note 4 - Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights-schedule-of-mortgage-servicing-rights-details</Role>
      <ShortName>Note 4 - Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>995502 - Disclosure - Note 4 - Mortgage Servicing Rights - Valuation Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-4-mortgage-servicing-rights-valuation-assumptions-details</Role>
      <ShortName>Note 4 - Mortgage Servicing Rights - Valuation Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>995503 - Disclosure - Note 5 - Derivatives (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-details-textual</Role>
      <ShortName>Note 5 - Derivatives (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-tables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>995504 - Disclosure - Note 5 - Derivatives - Schedule of Fair Value Hedges Cumulative-basis Adjustment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-schedule-of-fair-value-hedges-cumulativebasis-adjustment-details</Role>
      <ShortName>Note 5 - Derivatives - Schedule of Fair Value Hedges Cumulative-basis Adjustment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Note 5 - Derivatives - Schedule of Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-schedule-of-derivative-instruments-details</Role>
      <ShortName>Note 5 - Derivatives - Schedule of Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>995506 - Disclosure - Note 5 - Derivatives - Schedule of Hedging Relationship (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-schedule-of-hedging-relationship-details</Role>
      <ShortName>Note 5 - Derivatives - Schedule of Hedging Relationship (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>995507 - Disclosure - Note 5 - Derivatives - Schedule of Offsetting of Derivative Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-schedule-of-offsetting-of-derivative-assets-details</Role>
      <ShortName>Note 5 - Derivatives - Schedule of Offsetting of Derivative Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>995508 - Disclosure - Note 5 - Derivatives - Schedule of Offsetting of Derivative Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-5-derivatives-schedule-of-offsetting-of-derivative-liabilities-details</Role>
      <ShortName>Note 5 - Derivatives - Schedule of Offsetting of Derivative Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>995509 - Disclosure - Note 6 - Leases (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases-details-textual</Role>
      <ShortName>Note 6 - Leases (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases-tables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>995510 - Disclosure - Note 6 - Leases - Schedule of Lease Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases-schedule-of-lease-cost-details</Role>
      <ShortName>Note 6 - Leases - Schedule of Lease Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>995511 - Disclosure - Note 6 - Leases - Schedule of Supplemental Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases-schedule-of-supplemental-information-details</Role>
      <ShortName>Note 6 - Leases - Schedule of Supplemental Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>995512 - Disclosure - Note 6 - Leases - Schedule of Maturity for Operating Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-6-leases-schedule-of-maturity-for-operating-leases-details</Role>
      <ShortName>Note 6 - Leases - Schedule of Maturity for Operating Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>995513 - Disclosure - Note 7 - Deposits (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits-details-textual</Role>
      <ShortName>Note 7 - Deposits (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits-tables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>995514 - Disclosure - Note 7 - Deposits - Schedule of Deposit Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits-schedule-of-deposit-liabilities-details</Role>
      <ShortName>Note 7 - Deposits - Schedule of Deposit Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Note 7 - Deposits - Schedule of Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits-schedule-of-maturity-details</Role>
      <ShortName>Note 7 - Deposits - Schedule of Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>995516 - Disclosure - Note 7 - Deposits - Schedule Interest Expense by Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-7-deposits-schedule-interest-expense-by-category-details</Role>
      <ShortName>Note 7 - Deposits - Schedule Interest Expense by Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>995517 - Disclosure - Note 8 - Commitments and Contingencies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-8-commitments-and-contingencies-details-textual</Role>
      <ShortName>Note 8 - Commitments and Contingencies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-8-commitments-and-contingencies-tables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>995518 - Disclosure - Note 8 - Commitments and Contingencies - Schedule of Commitments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-8-commitments-and-contingencies-schedule-of-commitments-details</Role>
      <ShortName>Note 8 - Commitments and Contingencies - Schedule of Commitments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>995519 - Disclosure - Note 9 - Fair Value Measurements (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-details-textual</Role>
      <ShortName>Note 9 - Fair Value Measurements (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-tables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>995520 - Disclosure - Note 9 - Fair Value Measurements - Schedule of Financial Assets and Liabilities on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-schedule-of-financial-assets-and-liabilities-on-a-recurring-basis-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Schedule of Financial Assets and Liabilities on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>995521 - Disclosure - Note 9 - Fair Value Measurements - Schedule of Financial Assets on a Non-recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-schedule-of-financial-assets-on-a-nonrecurring-basis-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Schedule of Financial Assets on a Non-recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>995522 - Disclosure - Note 9 - Fair Value Measurements - Schedule of Schedule of Financial Assets and Liabilities on a Recurring and Non-recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-schedule-of-schedule-of-financial-assets-and-liabilities-on-a-recurring-and-nonrecurring-basis-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Schedule of Schedule of Financial Assets and Liabilities on a Recurring and Non-recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>995523 - Disclosure - Note 9 - Fair Value Measurements - Schedule of Reconciliation of Financial Assets and Liabilities with Unobservable Inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-schedule-of-reconciliation-of-financial-assets-and-liabilities-with-unobservable-inputs-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Schedule of Reconciliation of Financial Assets and Liabilities with Unobservable Inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>995524 - Disclosure - Note 9 - Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-9-fair-value-measurements-schedule-of-estimated-fair-values-of-financial-instruments-details</Role>
      <ShortName>Note 9 - Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>995525 - Disclosure - Note 10 - Earnings per Share - Schedule of Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-10-earnings-per-share-schedule-of-earnings-per-share-details</Role>
      <ShortName>Note 10 - Earnings per Share - Schedule of Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>995526 - Disclosure - Note 11 - Stock-based Compensation (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation-details-textual</Role>
      <ShortName>Note 11 - Stock-based Compensation (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation-tables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>995527 - Disclosure - Note 11 - Stock-based Compensation - Schedule of Stock Option Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation-schedule-of-stock-option-awards-details</Role>
      <ShortName>Note 11 - Stock-based Compensation - Schedule of Stock Option Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>995528 - Disclosure - Note 11 - Stock-based Compensation - Schedule of Stock Option Awards (Details) (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation-schedule-of-stock-option-awards-details-parentheticals</Role>
      <ShortName>Note 11 - Stock-based Compensation - Schedule of Stock Option Awards (Details) (Parentheticals)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>995529 - Disclosure - Note 11 - Stock-based Compensation - Schedule of Restricted Stock Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-11-stockbased-compensation-schedule-of-restricted-stock-awards-details</Role>
      <ShortName>Note 11 - Stock-based Compensation - Schedule of Restricted Stock Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>995530 - Disclosure - Note 12 - Regulatory Capital (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-12-regulatory-capital-details-textual</Role>
      <ShortName>Note 12 - Regulatory Capital (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-12-regulatory-capital-tables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>995531 - Disclosure - Note 12 - Regulatory Capital - Schedule of Comparison (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-12-regulatory-capital-schedule-of-comparison-details</Role>
      <ShortName>Note 12 - Regulatory Capital - Schedule of Comparison (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>995532 - Disclosure - Note 13 - Business Segments (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-13-business-segments-details-textual</Role>
      <ShortName>Note 13 - Business Segments (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-13-business-segments-tables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>995533 - Disclosure - Note 13 - Business Segments - Schedule of Segment Financial Results (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-13-business-segments-schedule-of-segment-financial-results-details</Role>
      <ShortName>Note 13 - Business Segments - Schedule of Segment Financial Results (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>995534 - Disclosure - Note 14 - Goodwill and Other Intangible Assets (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets-details-textual</Role>
      <ShortName>Note 14 - Goodwill and Other Intangible Assets (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets-tables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Note 14 - Goodwill and Other Intangible Assets - Schedule of Changes in Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets-schedule-of-changes-in-intangible-assets-details</Role>
      <ShortName>Note 14 - Goodwill and Other Intangible Assets - Schedule of Changes in Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>995536 - Disclosure - Note 14 - Goodwill and Other Intangible Assets - Schedule of Amortization (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-14-goodwill-and-other-intangible-assets-schedule-of-amortization-details</Role>
      <ShortName>Note 14 - Goodwill and Other Intangible Assets - Schedule of Amortization (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="fsbw20260630_10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>995537 - Disclosure - Note 15 - Definitive Agreement (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://investorrelations.fsbwa.com/20260630/role/statement-note-15-definitive-agreement-details-textual</Role>
      <ShortName>Note 15 - Definitive Agreement (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://investorrelations.fsbwa.com/20260630/role/statement-note-15-definitive-agreement</ParentRole>
      <Position>93</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>fsbw-20260630.xsd</File>
    <File>fsbw-20260630_cal.xml</File>
    <File>fsbw-20260630_def.xml</File>
    <File>fsbw-20260630_lab.xml</File>
    <File>fsbw-20260630_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="fsbw20260630_10q.htm">fsbw20260630_10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="3390">http://fasb.org/us-gaap/2026</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2026</BaseTaxonomy>
    <BaseTaxonomy items="5">http://xbrl.sec.gov/ecd/2026</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
