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SERVICING RIGHTS (Textual) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mortgage Banking [Member]    
Servicing Assets at Fair Value [Line Items]    
Servicing Portfolio $ 1,380.0fbspra_ServicingPortfolio
/ us-gaap_FinancialInstrumentAxis
= fbspra_MortgageBankingMember		 $ 1,370.0fbspra_ServicingPortfolio
/ us-gaap_FinancialInstrumentAxis
= fbspra_MortgageBankingMember
SBA [Member]    
Servicing Assets at Fair Value [Line Items]    
Servicing Portfolio $ 105.0fbspra_ServicingPortfolio
/ us-gaap_FinancialInstrumentAxis
= fbspra_SbaMember		 $ 114.4fbspra_ServicingPortfolio
/ us-gaap_FinancialInstrumentAxis
= fbspra_SbaMember