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Note 11 - Stock-based Compensation - Schedule of Stock Option Awards (Details) (Parentheticals) | 9 Months Ended
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Sep. 30, 2025
The 2018 Equity Incentive Plan [Member] |
Expected Forfeiture Rate | 0.31%

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- Definition Represents the share-based compensation arrangement by share-based payment award options expected forfeiture rate. + References No definition available. + Details
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Namespace Prefix: | fsbw_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |