Covid-19 Pandemic (Details) $ in Millions		3 Months Ended
Apr. 21, 2020 USD ($) item	Mar. 31, 2020
Staff approved to work remotely (as a percentage)		70.00%
Forecast
Secured approvals \| item	433
Fundings \| item	414
Loans funded	$ 72.0
One-to-four-family \| Payment Deferral \| Forecast
Loans restructured or modified	20.7
CONSUMER LOANS \| Payment Deferral \| Forecast
Loans restructured or modified	$ 8.2

The amount of loans funded under the Paycheck Protection Program ("PPE").

No definition available.

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

- Definition

Number of fundings in the Paycheck Protection Program ("PPP").

+ References

No definition available.

+ Details

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

- Definition

The percentage of staff who have been approved to work remotely.

+ References

No definition available.

+ Details

Name:	fsbw_PercentageOfStaffWorkingRemotely
Namespace Prefix:	fsbw_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of secured approvals in the Paycheck Protection Program ("PPP").

+ References

No definition available.

+ Details

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

- Definition

Amount of financing receivable modified on trial basis.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

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

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
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_LoanRestructuringModificationAxis=us-gaap_PaymentDeferralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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