Represents the amount established as A Percentage of Outstanding Credit Enhancement (CE) Obligation

No definition available.

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Period Type:	instant

- Definition

Represents the required credit enhancement (“CE”) obligation.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the credit enhancement (“CE”) obligation as a percentage of the outstanding loans.

+ References

No definition available.

+ Details

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Data Type:	dtr-types:percentItemType
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- Definition

Represents the amount established as an escrow holdback relative to the amount of outstanding credit obligation.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the establishment of a first loss account (“FLA”) related to loans.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of loans serviced as of balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of liabilities classified as other.

+ References

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

- Details

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- Details

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Note 9 - Commitments and Contingencies (Details Textual) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2023
Other Liabilities	$ 35,044,000			$ 35,737,000
Holdback Reserve For Guaranteed Loans [Member]
Other Liabilities	2,000,000			2,100,000
Federal Home Loan Bank of Des Moines [Member]
Loans Serviced	$ 8,700,000
Amount to Outstanding Credit Enhancement (CE) Obligation, Percentage	10.00%
Escrow Holdback Relative to Outstanding Credit Enhancement (CE) Obligation	$ 39,000
Federal Home Loan Bank of Des Moines [Member] \| Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Loans Serviced	0			0
Federal Home Loan Bank of Des Moines [Member] \| Recourse [Member]
First Loss Account (FLA)	581,000
Credit Enhancement (CE) Obligation	$ 389,000
Credit Enhancement (CE) Obligation, Percentage of Outstanding Loans	4.50%
Unfunded Loan Commitment [Member]
Financing Receivable, Allowance for Credit Loss	$ 1,500,000			$ 78,000,000,000
Financing Receivable, Allowance for Credit Loss, Recovery	$ 24,000	$ 149,000	$ 305,000