FAIR VALUE MEASUREMENTS - Discount Rate (Details)	Jun. 30, 2022	Dec. 31, 2021
Fair Value, Measurements, Recurring | Interest rate lock commitments with customers
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	fsbw:QuotedMarketPricesMember	fsbw:QuotedMarketPricesMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	fsbw:MeasurementInputPullThroughExpectationsMember	fsbw:MeasurementInputPullThroughExpectationsMember
Fair Value, Measurements, Recurring | Individual forward sale commitments with investors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	fsbw:QuotedMarketPricesMember	fsbw:QuotedMarketPricesMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	fsbw:MeasurementInputPullThroughExpectationsMember	fsbw:MeasurementInputPullThroughExpectationsMember
Fair Value, Measurements, Recurring | Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Fair Value, Measurements, Recurring | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	us-gaap:ValuationTechniqueDiscountedCashFlowMember	us-gaap:ValuationTechniqueDiscountedCashFlowMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	us-gaap:MeasurementInputDiscountRateMember	us-gaap:MeasurementInputDiscountRateMember
Fair Value, Measurements, Nonrecurring | Impaired Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	fsbw:FairValueOfUnderlyingCollateralMember	fsbw:FairValueOfUnderlyingCollateralMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	fsbw:MeasurementInputDiscountAppliedToObtainedAppraisalMember	fsbw:MeasurementInputDiscountAppliedToObtainedAppraisalMember
Fair Value, Measurements, Nonrecurring | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	fsbw:FairValueOfCollateralMember	fsbw:FairValueOfCollateralMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	fsbw:MeasurementInputDiscountAppliedToObtainedAppraisalMember	fsbw:MeasurementInputDiscountAppliedToObtainedAppraisalMember
Fair Value, Measurements, Nonrecurring | Servicing rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	fsbw:IndustrySourcesMember	fsbw:IndustrySourcesMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	fsbw:PrepaymentSpeedsMember	fsbw:PrepaymentSpeedsMember
Level 3 | Fair Value, Measurements, Recurring | Interest rate lock commitments with customers | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.80	0.80
Level 3 | Fair Value, Measurements, Recurring | Interest rate lock commitments with customers | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.99	0.99
Level 3 | Fair Value, Measurements, Recurring | Interest rate lock commitments with customers | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.975	0.933
Level 3 | Fair Value, Measurements, Recurring | Individual forward sale commitments with investors | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.80	0.80
Level 3 | Fair Value, Measurements, Recurring | Individual forward sale commitments with investors | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.99	0.99
Level 3 | Fair Value, Measurements, Recurring | Individual forward sale commitments with investors | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.975	0.933
Level 3 | Fair Value, Measurements, Recurring | Corporate securities | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.021
Level 3 | Fair Value, Measurements, Recurring | Corporate securities | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.027	0.027
Level 3 | Fair Value, Measurements, Recurring | Corporate securities | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.021	0.022
Level 3 | Fair Value, Measurements, Recurring | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.060	0.060
Level 3 | Fair Value, Measurements, Recurring | Municipal bonds | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.060	0.060
Level 3 | Fair Value, Measurements, Nonrecurring | Impaired Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.100	0.100
Level 3 | Fair Value, Measurements, Nonrecurring | Impaired Loans | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.100	0.100
Level 3 | Fair Value, Measurements, Nonrecurring | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.100	0.100
Level 3 | Fair Value, Measurements, Nonrecurring | OREO | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.100	0.100
Level 3 | Fair Value, Measurements, Nonrecurring | Servicing rights | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0	0
Level 3 | Fair Value, Measurements, Nonrecurring | Servicing rights | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.50	0.50
Level 3 | Fair Value, Measurements, Nonrecurring | Servicing rights | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	0.088	0.138