The amount of net allowance for credit loss.

No definition available.

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

- Definition

The amount of allowance for credit losses on unfunded loan commitments.

+ References

No definition available.

+ Details

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

- Definition

[Line Items] for Schedule of Accounting Policies [Table]

+ References

No definition available.

+ Details

Name:	fsbw_ScheduleOfAccountingPoliciesLineItems
Namespace Prefix:	fsbw_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL120267853-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2A
-URI http://asc.fasb.org/extlink&oid=123581744&loc=SL121698322-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL120267856-210455

+ Details

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

- Definition

Indicates (true false) whether practical expedient was elected to exclude accrued interest from amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2A
-URI http://asc.fasb.org/extlink&oid=123581744&loc=SL121698322-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL120267856-210455

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludedAccruedInterestFromAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, before allowance for credit loss, of accrued interest on investment in debt security measured at amortized cost (held-to-maturity).

+ References

+ Details

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

- Definition

Amount of allowance for credit loss for debt security measured at amortized cost (held-to-maturity), classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

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

- Definition

Indicates (true false) whether practical expedient was elected to exclude accrued interest from investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL120267963-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5C
-URI http://asc.fasb.org/extlink&oid=123581744&loc=SL121698325-111563

+ Details

Name:	us-gaap_DebtSecuritiesHeldToMaturityExcludedAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:booleanItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, after allowance for credit loss, of accrued interest on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 3A
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL120267960-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL120267845-210446

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=122038336&loc=d3e74567-122707

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=121614798&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

The cumulative amount of the reporting entity's undistributed earnings or deficit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(30)(a)(3))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 852
-SubTopic 10
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=84165509&loc=d3e56426-112766

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 65
-Paragraph 15
-Subparagraph (g)(2)
-URI http://asc.fasb.org/extlink&oid=121322162&loc=SL121327923-165333

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(23)(a)(4))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
-Paragraph 2
-Subparagraph (h)(2)
-URI http://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 65
-Paragraph 2
-Subparagraph (g)(2)(i)
-URI http://asc.fasb.org/extlink&oid=124501264&loc=SL117420844-207641

+ Details

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

- Details

Name:	us-gaap_InformationByCategoryOfDebtSecurityAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_OneToFourFamilyRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_MultiFamilyRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_ConsumerIndirectHomeImprovementLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_ConsumerLoansMarineLoanFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_ConsumerLoansOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_WarehouseLendingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fsbw_UnallocatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Adoption (Details) - USD ($)	Jun. 30, 2022	Jan. 01, 2022	Dec. 31, 2021
Schedule of Accounting Policies [Line Items]
Accrued interest receivable on available-for-sale debt securities	$ 1,200,000		$ 1,100,000
Accrued interest receivable on held to maturity debt securities	117,000		113,000
Accrued interest on loans	7,200,000
Increase in retained earnings	$ 189,075,000		179,215,000
Excluded accrued interest	true
Assets
Allowance for credit losses on loans	$ 24,967,000		25,635,000
Liabilities
Allowance for credit losses on unfunded loan commitments	3,400,000		$ 499,000
Securities available-for-sale.
Schedule of Accounting Policies [Line Items]
Accrued interest receivable on available-for-sale debt securities	$ 1,200,000
Accrued interest from amortized cost	true
ASU No 2016-13
Assets
Allowance for credit losses on loans		$ 25,635,000
Liabilities
Allowance for credit losses on unfunded loan commitments		499,000
ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Schedule of Accounting Policies [Line Items]
Increase in retained earnings	$ 297,000
Assets
Allowance for credit losses on debt securities held-to-maturity		72,000
Allowance for credit losses on loans		22,776,000
Liabilities
Allowance for credit losses on unfunded loan commitments		2,908,000
ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Schedule of Accounting Policies [Line Items]
Increase in retained earnings		297,000
Assets
Allowance for credit losses on debt securities held-to-maturity		72,000
Allowance for credit losses on loans		(2,859,000)
Liabilities
Allowance for credit losses on unfunded loan commitments		2,409,000
Total		(378,000)
Commercial \| ASU No 2016-13
Assets
Allowance for credit losses on loans		5,667,000
Commercial \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		1,728,000
Commercial \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		(3,939,000)
Construction and development \| ASU No 2016-13
Assets
Allowance for credit losses on loans		4,448,000
Construction and development \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		2,328,000
Construction and development \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		(2,120,000)
Home equity. \| ASU No 2016-13
Assets
Allowance for credit losses on loans		279,000
Home equity. \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		455,000
Home equity. \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		176,000
One-to-four-family \| ASU No 2016-13
Assets
Allowance for credit losses on loans		1,424,000
One-to-four-family \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		3,656,000
One-to-four-family \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		2,232,000
Multi-family \| ASU No 2016-13
Assets
Allowance for credit losses on loans		2,980,000
Multi-family \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		1,397,000
Multi-family \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		(1,583,000)
Indirect home improvement \| ASU No 2016-13
Assets
Allowance for credit losses on loans		3,540,000
Indirect home improvement \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		9,394,000
Indirect home improvement \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		5,854,000
Marine \| ASU No 2016-13
Assets
Allowance for credit losses on loans		702,000
Marine \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		900,000
Marine \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		198,000
Other consumer loans \| ASU No 2016-13
Assets
Allowance for credit losses on loans		38,000
Other consumer loans \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		64,000
Other consumer loans \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		26,000
Commercial and industrial \| ASU No 2016-13
Assets
Allowance for credit losses on loans		5,953,000
Commercial and industrial \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		2,727,000
Commercial and industrial \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		(3,226,000)
Warehouse lending \| ASU No 2016-13
Assets
Allowance for credit losses on loans		583,000
Warehouse lending \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjusted Balance
Assets
Allowance for credit losses on loans		127,000
Warehouse lending \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		(456,000)
Unallocated \| ASU No 2016-13
Assets
Allowance for credit losses on loans		21,000
Unallocated \| ASU No 2016-13 \| Cumulative Effect, Period of Adoption, Adjustment
Assets
Allowance for credit losses on loans		$ (21,000)