Fair Value Measurements - Discount Rate (Details)	Dec. 31, 2021	Dec. 31, 2020
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
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	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
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	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
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	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
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	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
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	Discount applied to the obtained appraisal
Fair Value, Measurements, Nonrecurring | Other Real Estate Owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-sale, Valuation Technique [Extensible List]	fsbw:FairValueOfUnderlyingCollateralMember
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	Discount applied to the obtained appraisal
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
Debt Securities, Available-for-sale, Measurement Input [Extensible List]	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)	93.3	91.6
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)	93.3	91.6
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)	2.2	2.5
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)	6.0	6.4
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)	10.0	10.0
Level 3 | Fair Value, Measurements, Nonrecurring | Impaired Loans | Discount applied to the obtained appraisal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	10.0
Level 3 | Fair Value, Measurements, Nonrecurring | Other Real Estate Owned | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)		10.0
Level 3 | Fair Value, Measurements, Nonrecurring | Other Real Estate Owned | Discount applied to the obtained appraisal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement inputs (as a percent)	10.0
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)	13.8	32.6