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SUBORDINATED DEBENTURES (Textual) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2013
installment
Debt Instrument [Line Items]      
Number of trusts created for issuing trust preferred securities 13fbspra_NumberOfTrustsCreatedForIssuingTrustPreferredSecurities    
Number of prior quarterly periods for deferral of cash interest payments on junior subordinated debentures before payment of all the cumulative deferred interest in March 2014     18fbspra_NumberOfQuarterlyPeriodsForDeferralOfCashInterestPaymentsOnJuniorSubordinatedDebt
Junior Subordinated Debt [Member]      
Debt Instrument [Line Items]      
Distributions accrued on junior subordinated debentures $ 3.0fbspra_DistributionsAccruedOnJuniorSubordinatedDebentures
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember		 $ 3.8fbspra_DistributionsAccruedOnJuniorSubordinatedDebentures
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember