OTHER REAL ESTATE OWNED (OREO) - Activity related to OREO (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Other Real Estate [Roll Forward]
Beginning balance	$ 145	$ 0	$ 0	$ 90
Additions			145
Gross proceeds from sale of OREO				(81)
Loss on sale of OREO				(9)
Ending balance	$ 145	$ 0	$ 145	$ 0

The amount of other real estate owned capitalized costs.

No definition available.

Name:	fsbw_OtherRealEstateOwnedCapitalizedCosts
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gross proceeds from the disposition of other real estate owned ("OREO").

+ References

No definition available.

+ Details

Name:	fsbw_OtherRealEstateOwnedDispositionOfAssets
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Carrying amount as of the balance sheet date of other real estate and foreclosed assets. Other real estate may include real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. Foreclosed assets include all assets obtained in full or partial satisfaction of a debt arrangement through foreclosure proceedings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Change in carrying amount of other real estate not separately disclosed. The change includes for example, but is not limited to, additions, improvements, disposals and valuation adjustments.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherRealEstatePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherRealEstateRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration