XML 53 R42.htm IDEA: XBRL DOCUMENT
v3.25.1

Note 2 - Investments - Schedule of Debt Securities, Held-to-Maturity Credit Quality Indicators (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Debt Securities, Held-to-Maturity, Amortized Cost | $ 10.5 | $ 8.5
Standard & Poor's, BBB/BBB Minus Rating [Member] |  |
Debt Securities, Held-to-Maturity, Amortized Cost | $ 10.5 | $ 8.5

X
- Definition Amount excluding accrued interest, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479344/326-20-45-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3B -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-5 + Details
Name: | us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | srt_CreditRatingStandardPoorsAxis=fsbw_StandardPoorsBBBOrBBBMinusRatingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |