Business Segments - Narrative (Details) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($) segment item	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments \| segment	2
Commercial and Consumer Banking	fsbw:CommercialAndConsumerBankingMember
Total deposits \| $	$ 1,446,269	$ 1,392,408
Retail Deposit \| Pacific Northwest
Segment Reporting Information [Line Items]
Number of bank branches \| item	21

Number of Bank Branches

No definition available.

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Namespace Prefix:	fsbw_
Data Type:	xbrli:integerItemType
Balance Type:	na
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=6876686&loc=d3e534808-122878

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Indicates business segment in which reporting unit is included.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=108376223&loc=SL108378252-109267

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=fsbw_PacificNorthwestMember
Namespace Prefix:
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Balance Type:
Period Type: