Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Note 4 - Loans Receivable and Allowance for Credit Losses on Loans - Schedule of Portfolio Loan Composition (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financing Receivable, Gross	$ 2,533,821	$ 2,433,015
ACL on loans	(31,870)	(31,534)	$ (27,992)	$ (25,635)
Total loans receivable, net	2,501,951	2,401,481
Real Estate Portfolio Segment [Member]
Financing Receivable, Gross	1,613,708	1,530,381
ACL on loans	(14,441)	(14,107)	(12,123)	(14,798)
Real Estate Portfolio Segment [Member] \| Commercial Real Estate [Member]
Financing Receivable, Gross	345,317	366,328
Real Estate Portfolio Segment [Member] \| Construction Loans [Member]
Financing Receivable, Gross	330,700	303,054
Real Estate Portfolio Segment [Member] \| Home Equity Loan [Member]
Financing Receivable, Gross	75,147	69,488
Real Estate Portfolio Segment [Member] \| One-to-four Family Loan [Member]
Financing Receivable, Gross	617,322	567,742
Real Estate Portfolio Segment [Member] \| Multifamily [Member]
Financing Receivable, Gross	245,222	223,769
Consumer Portfolio Segment [Member]
Financing Receivable, Gross	620,181	646,753
ACL on loans	(14,185)	(13,357)	(12,109)	(4,280)
Consumer Portfolio Segment [Member] \| Indirect Home Improvement Loan [Member]
Financing Receivable, Gross	541,946	569,903
Consumer Portfolio Segment [Member] \| Marine Loans [Member]
Financing Receivable, Gross	74,931	73,310
Consumer Portfolio Segment [Member] \| Other Consumer Loan [Member]
Financing Receivable, Gross	3,304	3,540
Commercial Business Portfolio Segment [Member]
Financing Receivable, Gross	299,932	255,881
ACL on loans	(3,244)	(4,070)	$ (3,760)	$ (6,536)
Commercial Business Portfolio Segment [Member] \| Commercial And Industrial [Member]
Financing Receivable, Gross	287,014	238,301
Commercial Business Portfolio Segment [Member] \| Warehouse Lending Loan [Member]
Financing Receivable, Gross	$ 12,918	$ 17,580