Name:	fsbw_CumulativeAmountOfFairValueHedgingAdjustmentIncludedInCarryingAmountOfHedgedAssetsLiabilities
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The amortized cost of closed portfolio used in the hedging relationships.

No definition available.

Name:	fsbw_DerivativeHedgingAmortizedCostBasisOfClosedPortfolios
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

The carrying amount of hedged assets (liabilities).

No definition available.

Name:	fsbw_HedgedAssetsLiabilitiesCarryingAmount
Namespace Prefix:	fsbw_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

Amount of hedged item in hedging relationship. Excludes hedged asset or liability.

No definition available.

Name:	us-gaap_DerivativeAmountOfHedgedItem
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

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.

No definition available.

Name:	us-gaap_DerivativeInstrumentsGainLossLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

Net fair value of all derivative instruments designated as fair value hedging instruments.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958