DEPOSITS - Deposit Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
DEPOSITS
Noninterest-bearing checking	$ 571,942	$ 564,360
Interest-bearing checking	158,607	228,024
Savings	156,313	193,922
Money market	680,246	552,357
Certificates of deposit less than $100,000	262,199	186,974
Certificates of deposit of $100,000 through $250,000	116,559	116,206
Certificates of deposit of $250,000 and over	53,812	57,512
Escrow accounts related to mortgages serviced	16,422	16,389
Total deposits	2,016,100	1,915,744
Interest-bearing domestic deposit, brokered deposits	1,200	90,000
Deposits, money market deposits, brokered deposits	78,800	5,000
Certificates of deposits less than 100000 brokered deposits	$ 180,300	97,600
Interest-bearing to noninterest-bearing due to reclass		$ 121,200

Deposits, Escrowed Accounts

No definition available.

Name:	fsbw_DepositsEscrowedAccounts
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of interest bearing checking balances reclassified to non-interest bearing checking balances for comparative purposes.

+ References

No definition available.

+ Details

Name:	fsbw_DepositsInterestBearingCheckingBalancesReclassifiedToNonInterestBearingCheckingBalances
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of deposits in accounts that offer many of the same services as checking accounts although transactions may be somewhat more limited, share some of the characteristics of a money market fund and are insured by the Federal government, also known as money market accounts placed by or through the assistance of a deposit broker

+ References

No definition available.

+ Details

Name:	fsbw_DepositsMoneyMarketDepositsBrokeredDeposits
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Time Deposits, $250,000 or More

+ References

No definition available.

+ Details

Name:	fsbw_TimeDeposits250000OrMore
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Time Deposits, between $100,000 and $250,000

+ References

No definition available.

+ Details

Name:	fsbw_TimeDepositsBetween100000And250000
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of time deposit liabilities, including certificates of deposit, placed by or through the assistance of a deposit broker, in denominations of less than $100,000.

+ References

No definition available.

+ Details

Name:	fsbw_TimeDepositsLessThan100000BrokeredDeposits
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_Deposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DepositsByTypeAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

+ References

+ Details

Name:	us-gaap_DepositsMoneyMarketDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of interest bearing deposits with no stated maturity, which may include passbook and statement savings accounts and money-market deposit accounts (MMDAs).

+ References

+ Details

Name:	us-gaap_DepositsSavingsDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of interest-bearing domestic third-party deposits placed by or through the assistance of a deposit broker, also known as brokered deposit liabilities.

+ References

+ Details

Name:	us-gaap_InterestBearingDomesticDepositBrokered
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of interest-bearing domestic checking deposit liabilities.

+ References

+ Details

Name:	us-gaap_InterestBearingDomesticDepositChecking
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of noninterest-bearing domestic checking deposit liabilities.

+ References

+ Details

Name:	us-gaap_NoninterestBearingDomesticDepositChecking
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations of less than $100,000.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher SEC
-Name Industry Guide
-Number 3
-Section V
-Paragraph D-E

+ Details

Name:	us-gaap_TimeDepositsLessThan100000
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant