Represents the deposit premium for core deposits as a percentage.

No definition available.

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Namespace Prefix:	fsbw_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage paid for public funds in a business combination.

+ References

No definition available.

+ Details

Name:	fsbw_BusinessCombinationPublicFundsPercentage
Namespace Prefix:	fsbw_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of deposit liabilities assumed at the acquisition date.

+ References

No definition available.

+ Details

Name:	fsbw_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesDepositsTotal
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The goodwill amortization period for deductible tax purposes.

+ References

No definition available.

+ Details

Name:	fsbw_GoodwillAmorizationPeriodForDeductibleTaxPurposes
Namespace Prefix:	fsbw_
Data Type:	xbrli:durationItemType
Balance Type:	na
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- Definition

The number of businesses acquired by the entity during the period.

+ References

No definition available.

+ Details

Name:	fsbw_NumberOfBusinessesAcquiredBankOwnedBranches
Namespace Prefix:	fsbw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of leases acquired.

+ References

No definition available.

+ Details

Name:	fsbw_NumberOfBusinessesAcquiredLeasesAcquiredWithBankBranches
Namespace Prefix:	fsbw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the number of financial assets purchased with credit deterioration.

+ References

No definition available.

+ Details

Name:	fsbw_NumberOfFinancialAssetsPurchasedWithCreditDeterioration
Namespace Prefix:	fsbw_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the best estimate at the acquisition date of the contractual cash flows not expected to be collected, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(3)
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-URI https://asc.fasb.org/1943274/2147479907/805-20-50-1

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of acquired receivable from business combination, excluding certain loans and debt securities acquired in transfer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
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Period Type:	instant

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the gross contractual amounts receivable, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-SubTopic 20
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-Subparagraph (b)(2)
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The cash inflow associated with the acquisition of business during the period (for example, cash that was held by the acquired business).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
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-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-12

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, after accumulated impairment loss, of asset representing future economic benefit arising from other asset acquired in business combination or from joint venture formation or both, that is not individually identified and separately recognized.

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Reference 1: http://www.xbrl.org/2003/role/exampleRef
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The number of businesses acquired by the entity during the period.

+ References

No definition available.

+ Details

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

- Definition

The cash outflow associated with the acquisition of business during the period. The cash portion only of the acquisition price.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 30
-Name Accounting Standards Codification
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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=fsbw_ColumbiaStateBankMember
Namespace Prefix:
Data Type:	na
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Period Type:

- Details

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Note 2 - Business Combination (Details Textual) $ in Thousands		12 Months Ended
	Feb. 24, 2023 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Cash Acquired from Acquisition		$ 0	$ 336,157	$ 0
Goodwill		$ 3,592	$ 3,592
Columbia State Bank [Member]
Number of Businesses Acquired	7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities, Deposits, Total	$ 424,917
Number of Businesses Acquired, Bank Owned Branches	6
Number of Businesses Acquired, Leases Acquired with Bank Branches	1
Business Combination, Core Deposits Premium, Percentage	4.15%
Business Combination, Public Funds, Percentage	2.50%
Cash Acquired from Acquisition	$ 334,700
Number of Financial Assets Purchased with Credit Deterioration	0
Business Combination, Acquired Receivable, Fair Value	$ 63,200
Business Combination, Acquired Receivables, Gross Contractual Amount	66,100
Business Combination, Acquired Receivables, Estimated Uncollectible	0
Goodwill	$ 1,280
Columbia State Bank [Member] \| Core Deposits [Member]
Finite-Lived Intangible Asset, Useful Life (Year)	10 years	10 years
Columbia State Bank [Member] \| Workforce and Customer Relationships [Member]
Goodwill, Amorization Period for Deductible Tax Purposes (Year)	15 years
Columbia State Bank [Member] \| Business Combination, Payment for Loans Acquired [Member]
Payments to Acquire Businesses, Gross	$ 66,600
Columbia State Bank [Member] \| Business Combination, Payment for Bank Facilities and Other Assets [Member]
Payments to Acquire Businesses, Gross	6,300
Columbia State Bank [Member] \| Business Combination, Payment for Core Deposits and Deposits Assumed [Member]
Payments to Acquire Businesses, Gross	16,400
Columbia State Bank [Member] \| Reported Value by Acquiree [Member]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities, Deposits, Total	425,465
Goodwill	$ 0