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Note 9 - Deposits (Details Textual) - USD ($) | Mar. 31, 2024 | Dec. 31, 2023
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Deposit Liabilities, Checking [Member] |  |
Interest-Bearing Domestic Deposit, Brokered | $ 0 | $ 70,200,000
Deposit Liabilities, Money Market [Member] |  |
Interest-Bearing Domestic Deposit, Brokered | 8,000,000 | 1,000
Deposit Liabilities, Certificates of Deposits [Member] |  |
Interest-Bearing Domestic Deposit, Brokered | $ 331,300,000 | $ 361,300,000

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- Definition Amount of interest-bearing domestic third-party deposits placed by or through the assistance of a deposit broker, also known as brokered deposit liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_FairValueByLiabilityClassAxis=fsbw_DepositLiabilitiesCheckingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByLiabilityClassAxis=fsbw_DepositLiabilitiesMoneyMarketMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByLiabilityClassAxis=fsbw_DepositLiabilitiesCertificatesOfDepositsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |