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Note 8 - Other Real Estate Owned ("OREO") (Details Textual) - Real Estate Owned (OREO) [Member] | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2022 USD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($)
Number of Real Estate Properties | 1 | 0 | 0
Custody Fees | $ 10,000 |  |
Mortgage Loans in Process of Foreclosure, Amount |  | $ 84,000 | $ 96,000

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- Definition Those fees charged for services related to holding, advising, and managing customer investment assets. These account fees usually charged annually or semi-annually, on a per security basis, for the costs of services. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03(3)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483621/220-10-S99-2 + Details
Name: | us-gaap_CustodyFees
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 310 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 35 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481962/310-10-50-35 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The number of real estate properties owned as of the balance sheet date. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_RealEstatePropertiesAxis=fsbw_RealEstateOwnedOREOMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |