<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>698</ContextCount>
  <ElementCount>576</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>104</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>14</UnitCount>
  <MyReports>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - Consolidated Balance Sheets (Parentheticals)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedBalanceSheetsParentheticals</Role>
      <ShortName>Consolidated Balance Sheets (Parentheticals)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedStatementsOfIncome</Role>
      <ShortName>Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00300 - Statement - Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00400 - Statement - Consolidated Statements of Changes in Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00405 - Statement - Consolidated Statements of Changes in Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedStatementsOfChangesInStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>00500 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Loans Receivable and Allowance For Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLosses</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Servicing Rights</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureServicingRights</Role>
      <ShortName>Servicing Rights</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDerivatives</Role>
      <ShortName>Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Other Real Estate Owned</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureOtherRealEstateOwned</Role>
      <ShortName>Other Real Estate Owned</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDeposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Employee Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEmployeeBenefits</Role>
      <ShortName>Employee Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11301 - Disclosure - Stock-Based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureStockBasedCompensation</Role>
      <ShortName>Stock-Based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>11401 - Disclosure - Regulatory Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureRegulatoryCapital</Role>
      <ShortName>Regulatory Capital</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>11501 - Disclosure - Business Segments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBusinessSegments</Role>
      <ShortName>Business Segments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>11601 - Disclosure - Goodwill And Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureGoodwillAndOtherIntangibleAssets</Role>
      <ShortName>Goodwill And Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>11701 - Disclosure - Revenue From Contracts With Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureRevenueFromContractsWithCustomers</Role>
      <ShortName>Revenue From Contracts With Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>11801 - Disclosure - COVID-19 Pandemic</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCovid19Pandemic</Role>
      <ShortName>COVID-19 Pandemic</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>20102 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>30203 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureInvestments</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>30303 - Disclosure - Loans Receivable and Allowance For Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesTables</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLosses</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Servicing Rights (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureServicingRightsTables</Role>
      <ShortName>Servicing Rights (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureServicingRights</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureDerivatives</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureLeases</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Other Real Estate Owned (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureOtherRealEstateOwnedTables</Role>
      <ShortName>Other Real Estate Owned (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureOtherRealEstateOwned</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureDeposits</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>30903 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingencies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>31103 - Disclosure - Employee Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEmployeeBenefitsTables</Role>
      <ShortName>Employee Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureEmployeeBenefits</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>31203 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureEarningsPerShare</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>31303 - Disclosure - Stock-Based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureStockBasedCompensationTables</Role>
      <ShortName>Stock-Based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureStockBasedCompensation</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>31503 - Disclosure - Business Segments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBusinessSegmentsTables</Role>
      <ShortName>Business Segments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureBusinessSegments</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>31603 - Disclosure - Goodwill And Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill And Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureGoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>31703 - Disclosure - Revenue From Contracts With Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureRevenueFromContractsWithCustomersTables</Role>
      <ShortName>Revenue From Contracts With Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureRevenueFromContractsWithCustomers</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Basis of Presentation and Summary of Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>Basis of Presentation and Summary of Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureBasisOfPresentationAndSummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Investments - Available-for-sale securities reconciliation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestmentsAvailableForSaleSecuritiesReconciliationDetails</Role>
      <ShortName>Investments - Available-for-sale securities reconciliation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40202 - Disclosure - Investments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestmentsNarrativeDetails</Role>
      <ShortName>Investments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40203 - Disclosure - Investments - Investments with Unrealized Losses Policy (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestmentsInvestmentsWithUnrealizedLossesPolicyDetails</Role>
      <ShortName>Investments - Investments with Unrealized Losses Policy (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40204 - Disclosure - Investments - Available for Sale Securities by Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestmentsAvailableForSaleSecuritiesByContractualMaturityDetails</Role>
      <ShortName>Investments - Available for Sale Securities by Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40205 - Disclosure - Investments - Gains and Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureInvestmentsGainsAndLossesDetails</Role>
      <ShortName>Investments - Gains and Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Loans Receivable and Allowance For Loan Losses - Composition of Loan Portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesCompositionOfLoanPortfolioDetails</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses - Composition of Loan Portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>40302 - Disclosure - Loans Receivable and Allowance For Loan Losses - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesNarrativeDetails</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40303 - Disclosure - Loans Receivable and Allowance For Loan Losses - Allowance for Loan Losses by Loan Categories (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesAllowanceForLoanLossesByLoanCategoriesDetails</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses - Allowance for Loan Losses by Loan Categories (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40304 - Disclosure - Loans Receivable and Allowance For Loan Losses - Aging Analysis of Past Due Loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesAgingAnalysisOfPastDueLoansDetails</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses - Aging Analysis of Past Due Loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40305 - Disclosure - Loans Receivable and Allowance For Loan Losses - Financing Receivables, Related Allowance Recorded and No Related Allowance Recorder (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesFinancingReceivablesRelatedAllowanceRecordedAndNoRelatedAllowanceRecorderDetails</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses - Financing Receivables, Related Allowance Recorded and No Related Allowance Recorder (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40306 - Disclosure - Loans Receivable and Allowance For Loan Losses - Loans by Credit Quality Indicator (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLoansReceivableAndAllowanceForLoanLossesLoansByCreditQualityIndicatorDetails</Role>
      <ShortName>Loans Receivable and Allowance For Loan Losses - Loans by Credit Quality Indicator (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Servicing Rights - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureServicingRightsNarrativeDetails</Role>
      <ShortName>Servicing Rights - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40402 - Disclosure - Servicing Rights - Servicing Rights (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureServicingRightsServicingRightsDetails</Role>
      <ShortName>Servicing Rights - Servicing Rights (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40403 - Disclosure - Servicing Rights - Valuation Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureServicingRightsValuationAssumptionsDetails</Role>
      <ShortName>Servicing Rights - Valuation Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>40404 - Disclosure - Servicing Rights - Changes in Valuation Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureServicingRightsChangesInValuationAssumptionsDetails</Role>
      <ShortName>Servicing Rights - Changes in Valuation Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Derivatives (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDerivativesDetails</Role>
      <ShortName>Derivatives (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureDerivativesTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLeasesDetails</Role>
      <ShortName>Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureLeasesTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>40602 - Disclosure - Leases - Components of lease cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLeasesComponentsOfLeaseCostDetails</Role>
      <ShortName>Leases - Components of lease cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>40603 - Disclosure - Leases - Maturities of operating lease liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureLeasesMaturitiesOfOperatingLeaseLiabilitiesDetails</Role>
      <ShortName>Leases - Maturities of operating lease liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Other Real Estate Owned - Activity related to OREO (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureOtherRealEstateOwnedActivityRelatedToOreoDetails</Role>
      <ShortName>Other Real Estate Owned - Activity related to OREO (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>40702 - Disclosure - Other Real Estate Owned - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureOtherRealEstateOwnedNarrativeDetails</Role>
      <ShortName>Other Real Estate Owned - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Deposits - Deposit Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDepositsDepositLiabilitiesDetails</Role>
      <ShortName>Deposits - Deposit Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>40802 - Disclosure - Deposits - Maturities of Time Deposits for Future Periods (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDepositsMaturitiesOfTimeDepositsForFuturePeriodsDetails</Role>
      <ShortName>Deposits - Maturities of Time Deposits for Future Periods (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>40803 - Disclosure - Deposits - Interest Expense by Deposit Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureDepositsInterestExpenseByDepositCategoryDetails</Role>
      <ShortName>Deposits - Interest Expense by Deposit Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Commitments and Contingencies - Commitment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingenciesCommitmentDetails</Role>
      <ShortName>Commitments and Contingencies - Commitment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>40902 - Disclosure - Commitments and Contingencies - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingenciesNarrativeDetails</Role>
      <ShortName>Commitments and Contingencies - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>40903 - Disclosure - Commitments and Contingencies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingenciesNarrativeDetailss</Role>
      <ShortName>Commitments and Contingencies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureCommitmentsAndContingenciesTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Fair Value Measurements - Available for Sale Securities Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurementsAvailableForSaleSecuritiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Available for Sale Securities Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>41006 - Disclosure - Fair Value Measurements - Impaired Loans and OREO (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurementsImpairedLoansAndOreoDetails</Role>
      <ShortName>Fair Value Measurements - Impaired Loans and OREO (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>41007 - Disclosure - Fair Value Measurements - Discount Rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurementsDiscountRateDetails</Role>
      <ShortName>Fair Value Measurements - Discount Rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>41008 - Disclosure - Fair Value Measurements - Fair Value Level 3 on recurring basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurementsFairValueLevel3OnRecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Fair Value Level 3 on recurring basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>41009 - Disclosure - Fair Value Measurements - Fair Value By Balance Sheet Grouping (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureFairValueMeasurementsFairValueByBalanceSheetGroupingDetails</Role>
      <ShortName>Fair Value Measurements - Fair Value By Balance Sheet Grouping (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Employee Benefits - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEmployeeBenefitsNarrativeDetails</Role>
      <ShortName>Employee Benefits - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>41102 - Disclosure - Employee Benefits - Schedule of Shares Under ESOP (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEmployeeBenefitsScheduleOfSharesUnderEsopDetails</Role>
      <ShortName>Employee Benefits - Schedule of Shares Under ESOP (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>41201 - Disclosure - Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureEarningsPerShareScheduleOfEarningsPerShareBasicAndDilutedDetails</Role>
      <ShortName>Earnings Per Share - Schedule of Earnings Per Share, Basic and Diluted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>41301 - Disclosure - Stock-Based Compensation - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureStockBasedCompensationNarrativeDetails</Role>
      <ShortName>Stock-Based Compensation - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>41303 - Disclosure - Stock-Based Compensation - Stock Option Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureStockBasedCompensationStockOptionActivityDetails</Role>
      <ShortName>Stock-Based Compensation - Stock Option Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>41304 - Disclosure - Stock-Based Compensation - Restricted Stock Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureStockBasedCompensationRestrictedStockAwardsDetails</Role>
      <ShortName>Stock-Based Compensation - Restricted Stock Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>41401 - Disclosure - Regulatory Capital - Compliance with Regulatory Capital Requirements under Banking Regulations (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureRegulatoryCapitalComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsDetails</Role>
      <ShortName>Regulatory Capital - Compliance with Regulatory Capital Requirements under Banking Regulations (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>41501 - Disclosure - Business Segments - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBusinessSegmentsNarrativeDetails</Role>
      <ShortName>Business Segments - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>41502 - Disclosure - Business Segments - Segment Financial Results (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureBusinessSegmentsSegmentFinancialResultsDetails</Role>
      <ShortName>Business Segments - Segment Financial Results (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>41601 - Disclosure - Goodwill And Other Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureGoodwillAndOtherIntangibleAssetsDetails</Role>
      <ShortName>Goodwill And Other Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureGoodwillAndOtherIntangibleAssetsTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>41701 - Disclosure - Revenue From Contracts With Customers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureRevenueFromContractsWithCustomersDetails</Role>
      <ShortName>Revenue From Contracts With Customers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fsbwa.com/role/DisclosureRevenueFromContractsWithCustomersTables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="fsbw-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>41801 - Disclosure - Covid-19 Pandemic (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fsbwa.com/role/DisclosureCovid19PandemicDetails</Role>
      <ShortName>Covid-19 Pandemic (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</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-20200331.xml</File>
    <File>fsbw-20200331.xsd</File>
    <File>fsbw-20200331_cal.xml</File>
    <File>fsbw-20200331_def.xml</File>
    <File>fsbw-20200331_lab.xml</File>
    <File>fsbw-20200331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
