Note 2 - Investment Securities (Details)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Note 2 - Investment Securities (Details) [Line Items]
Investment securities available-for-sale, at fair value	$ 248,294,725	$ 292,298,910
US Government Agencies Debt Securities [Member]
Note 2 - Investment Securities (Details) [Line Items]
Available-for-sale Securities Pledged as Collateral	11,800,000	9,700,000
Held-to-maturity Securities Pledged as Collateral	$ 508,000	507,000
Investment securities available-for-sale, at fair value		31,232,000
US States and Political Subdivisions Debt Securities [Member]
Note 2 - Investment Securities (Details) [Line Items]
Number of Investment Securities	56
Investment securities available-for-sale, at fair value	$ 56,831,000	$ 55,702,000
Municipal General Obligation Bonds [Member]
Note 2 - Investment Securities (Details) [Line Items]
Investment securities available-for-sale, at fair value	35,135,000
Municipal General Obligation Bonds [Member] \| WASHINGTON
Note 2 - Investment Securities (Details) [Line Items]
Investment securities available-for-sale, at fair value	3,436,000
Municipal General Obligation Bonds [Member] \| Insurance Policy Assured Guaranty Municipal Corp [Member] \| King County, Washington [Member] \| WASHINGTON
Note 2 - Investment Securities (Details) [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,400,000
Investment securities available-for-sale, at fair value	2,400,000
Municipal Revenue Bonds [Member]
Note 2 - Investment Securities (Details) [Line Items]
Investment securities available-for-sale, at fair value	21,696,000
Municipal Revenue Bonds [Member] \| NEW YORK
Note 2 - Investment Securities (Details) [Line Items]
Investment securities available-for-sale, at fair value	7,531,000
Municipal Revenue Bonds [Member] \| Dormitory Authority of the State of New York [Member] \| NEW YORK
Note 2 - Investment Securities (Details) [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,900,000
Investment securities available-for-sale, at fair value	$ 3,000,000
Percent of State Income Tax Used to Secure Debt	25.00%

No definition available.

Name:	fsbk_Note2InvestmentSecuritiesDetailsLineItems
Namespace Prefix:	fsbk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of investment securities held by the entity.

+ References

No definition available.

+ Details

Name:	fsbk_NumberOfInvestmentSecurities
Namespace Prefix:	fsbk_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The percent of state personal income tax used to secure the debt.

+ References

No definition available.

+ Details

Name:	fsbk_PercentOfStateIncomeTaxUsedToSecureDebt
Namespace Prefix:	fsbk_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5086-111524

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of held-to-maturity securities that serve as collateral for borrowings.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesPledgedAsCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fsbk_MunicipalGeneralObligationBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_WA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=fsbk_InsurancePolicyAssuredGuarantyMunicipalCorpMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=fsbk_KingCountyWashingtonMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fsbk_MunicipalRevenueBondsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementGeographicalAxis=stpr_NY
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CounterpartyNameAxis=fsbk_DormitoryAuthorityOfTheStateOfNewYorkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: