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Note 11 - Debt - Summary of Advances From Federal Home Loan Bank (Details) - FHLB and FRB BTFP [Member] $ in Thousands	Dec. 31, 2023 USD ($)
2024, balances	$ 93,746
2024, interest rates	4.60%

X

- Definition

Amount of debt payable, sinking fund requirement, and other securities issued that are redeemable by holder at fixed or determinable price and date, maturing in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

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- Definition

Weighted average interest rate of advance due in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

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- Details

Name:	us-gaap_DebtInstrumentAxis=fsbw_FHLBAndFRBBTFBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: