Note 9 - Deposits - Schedule Interest Expense by Category (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Interest-bearing checking	$ 643	$ 100	$ 1,111	$ 353
Savings and money market	1,529	1,303	4,071	2,397
Certificates of deposit	8,290	1,193	19,514	2,688
Total	$ 10,462	$ 2,596	$ 24,696	$ 5,438

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//1943274/2147483589/942-220-S99-1

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest expense on domestic certificates of deposit liabilities.

+ References

+ Details

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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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Data Type:	xbrli:monetaryItemType
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