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Note 3 - Loans Receivable and Allowance for Credit Losses on Loans - Loan Modifications to Borrowers Experiencing Financial Difficulty (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Payment Deferral [Member] | Commercial Real Estate Portfolio Segment [Member]
Amortized cost basis	$ 1,116	$ 1,088
% of total loan type	0.30%	0.30%
Weighted-average period (Year)	2 years	1 year 6 months
Commercial And Industrial [Member] | Extended Maturity and Interest Rate Reduction [Member] | Commercial Business Portfolio Segment [Member]
Amortized cost basis		$ 2,940
% of total loan type		1.20%
Weighted-average period (Year)		5 years
Pre-modification %		7.50%
Post-modification %		4.10%

X

- Definition

Percentage of original weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

No definition available.

+ Details

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

X

- Definition

Percentage of post-modification weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

No definition available.

+ Details

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Namespace Prefix:	fsbw_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
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-Publisher FASB
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+ Details

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X

- Definition

Accumulated amortized cost of financing receivable modified for debtor experiencing financial difficulty.

+ References

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-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
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-Paragraph 42
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-Publisher FASB
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Balance Type:	debit
Period Type:	instant

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

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- Details

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- Details

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- Details

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- Details

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