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Note 16 - Junior Subordinated Debentures (Details) - Consolidated Debt Obligations Related to a Subsidiary Trust Holding (USD $)
Mar. 31, 2015
Note 16 - Junior Subordinated Debentures (Details) - Consolidated Debt Obligations Related to a Subsidiary Trust Holding [Line Items]  
Junior subordinated debentures owed to unconsolidated subsidiary trust $ 10,310,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
First South Preferred Trust I [Member]  
Note 16 - Junior Subordinated Debentures (Details) - Consolidated Debt Obligations Related to a Subsidiary Trust Holding [Line Items]  
Junior subordinated debentures owed to unconsolidated subsidiary trust 10,000,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
/ us-gaap_DebtInstrumentAxis
= fsbk_FirstSouthPreferredTrustIMember
Increase First South Preferred Trust I [Member]  
Note 16 - Junior Subordinated Debentures (Details) - Consolidated Debt Obligations Related to a Subsidiary Trust Holding [Line Items]  
Junior subordinated debentures owed to unconsolidated subsidiary trust $ 310,000us-gaap_JuniorSubordinatedDebentureOwedToUnconsolidatedSubsidiaryTrust
/ us-gaap_DebtInstrumentAxis
= fsbk_IncreaseFirstSouthPreferredTrustIMember