Represents the share-based compensation arrangement by share-based payment award, award vesting percentage.

No definition available.

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- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Balance Type:	debit
Period Type:	duration

- Definition

Amount of cost to be recognized for option under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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Period Type:	instant

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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- Definition

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI https://asc.fasb.org/extlink&oid=128089324&loc=d3e5070-113901

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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Balance Type:
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- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_StockOptionMember
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- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=fsbw_EquityOptionAndRestrictedStockMember
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- Details

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Balance Type:
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- Details

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- Details

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- Details

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STOCK-BASED COMPENSATION - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022	May 17, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award contractual life	10 years
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 654,000	$ 451,000
Expected term in years			6 years 10 months 2 days
Remaining weighted-average vesting period	6 years 7 months 2 days
2018 Equity Incentive Plan \| Equity option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award contractual life	10 years
Shares available for grant	356,532
Unrecognized compensation cost, nonvested awards	$ 1,600,000
Remaining weighted-average vesting period	3 years 1 month 6 days
2018 Equity Incentive Plan \| Restricted stock awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				326,000
Shares available for grant	114,222
Unrecognized compensation cost, nonvested awards	$ 2,400,000
Weighted-average vesting period	3 years
2018 Equity Incentive Plan \| Equity option and restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				1,300,000
Maximum \| 2018 Equity Incentive Plan \| Restricted stock awards \| Independent Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Maximum \| 2018 Equity Incentive Plan \| Restricted stock awards \| Officers And Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Minimum \| 2018 Equity Incentive Plan \| Restricted stock awards \| Independent Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
Minimum \| 2018 Equity Incentive Plan \| Restricted stock awards \| Officers And Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	2 years
One year vesting \| 2018 Equity Incentive Plan \| Equity option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term in years	5 years 6 months
One year vesting \| 2018 Equity Incentive Plan \| Equity option \| Directors Excluding CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
One year vesting \| Minimum \| 2018 Equity Incentive Plan \| Equity option \| Directors Excluding CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	1 year
Two year vesting \| 2018 Equity Incentive Plan \| Equity option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual award vesting percentage	20.00%
Expected term in years	5 years 9 months
Two year vesting \| 2018 Equity Incentive Plan \| Equity option \| Directors Excluding CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	2 years
Two year vesting \| Maximum \| 2018 Equity Incentive Plan \| Equity option \| Officers And Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Two year vesting \| Minimum \| 2018 Equity Incentive Plan \| Equity option \| Officers And Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	2 years
Three year vesting \| 2018 Equity Incentive Plan \| Equity option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term in years	6 years
Three year vesting \| 2018 Equity Incentive Plan \| Equity option \| Directors Excluding CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Three year vesting \| Maximum \| 2018 Equity Incentive Plan \| Equity option \| Directors Excluding CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Four year vesting \| 2018 Equity Incentive Plan \| Equity option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term in years	6 years 6 months
Four year vesting \| 2018 Equity Incentive Plan \| Equity option \| Directors Excluding CEO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years