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Note 3 - Loans Receivable and Allowance for Credit Losses - Loans - Schedule of Modified Financing Receivables (Details) - Commercial Real Estate Portfolio Segment [Member] - Commercial Loan [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Financing Receivable, Excluding Accrued Interest, Modified in Period, to Total Financing Receivables, Percentage	0.70%	0.60%
Extended Maturity [Member]
Financing Receivable, Excluding Accrued Interest, Modified, Weighted Average Term Extension Payment Delay (Year)	2 years 8 months 12 days	1 year 8 months 12 days
Extended Maturity and Payment Delay [Member]
Financing Receivable, Excluding Accrued Interest, Modified in Period, Amount	$ 1,196	$ 1,102

X

- Definition

Period of increase in weighted average maturity of financing receivable modified (excluding accrued interest) for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days

+ References

No definition available.

+ Details

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Namespace Prefix:	fsbw_
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Balance Type:	na
Period Type:	duration

X

- Definition

Amortized cost excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

+ Details

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

X

- Definition

Percentage, excluding accrued interest, of financing receivable modified during period for debtor experiencing financial difficulty to total financing receivables.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-42

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- Details

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- Details

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- Details

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- Details

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