Note 9 - Deposits - Schedule Interest Expense by Category (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Interest-bearing checking	$ 784	$ 98
Savings and money market	1,661	1,198
Certificates of deposit	10,437	5,328
Total	$ 12,882	$ 6,624

The aggregate interest expense incurred on the following for deposits in domestic offices: (1) money market deposits and (2) savings deposits.

No definition available.

+ Details

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Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate amount of interest expense on all deposits.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

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

- Definition

Amount of interest expense on domestic certificates of deposit liabilities.

+ References

+ Details

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

- Definition

Amount of interest expense on domestic checking deposit liabilities.

+ References

+ Details

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