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Note 14 - Significant Concentration of Credit Risk (Details Textual)	12 Months Ended
	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%

X

- 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

X

- Details

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

X

- Details

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