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Note 12 - Stock-based Compensation - Schedule of Stock Option Awards (Details) (Parentheticals)	6 Months Ended
	Jun. 30, 2024
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_
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Balance Type:	na
Period Type:	duration

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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: