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Note 14 - Significant Concentration of Credit Risk (Details Textual) | 12 Months Ended
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Dec. 31, 2024
Construction Land Development and Other Land Concentration [Member] |
Concentration Risk, Percentage of Total Regulatory Capital | 89.80%
Commercial Real Estate Portfolio Segment [Member] |
Concentration Risk, Percentage of Total Regulatory Capital | 256.80%

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- Definition Amount of concentration risk as percentage of total regulatory capital. + References No definition available. + Details
Name: | fsbw_ConcentrationRiskPercentageOfTotalRegulatoryCapital
Namespace Prefix: | fsbw_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_ConstructionLandDevelopmentAndOtherLandConcentrationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |