Note 8 - Other Real Estate Owned ("OREO") (Details Textual) - Real Estate Owned (OREO) [Member]	12 Months Ended
Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Number of Real Estate Properties	0	1	0
Custody Fees	$ 0	$ 10,000	$ 0
Mortgage Loans in Process of Foreclosure, Amount	$ 96,000	$ 511,000

Those fees charged for services related to holding, advising, and managing customer investment assets. These account fees usually charged annually or semi-annually, on a per security basis, for the costs of services.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

+ Details

Name:	us-gaap_CustodyFees
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 35
-SubTopic 10
-Topic 310
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-35

+ Details

Name:	us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The number of real estate properties owned as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	us-gaap_NumberOfRealEstateProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_RealEstatePropertiesAxis=fsbw_RealEstateOwnedOREOMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: