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

X

- 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

X

- Details

Name:	us-gaap_PlanNameAxis=fsbw_The2018EquityIncentivePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: