Note 11 - Stock-based Compensation - Schedule of Stock Option Awards (Details) (Parentheticals)	6 Months Ended
Jun. 30, 2026
Expected to vest, options, expected forfeiture rate	0.31%

Represents the share-based compensation arrangement by share-based payment award options expected forfeiture rate.

No definition available.

Name:	fsbw_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExpectedForfeitureRate
Namespace Prefix:	fsbw_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration