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LOANS AND ALLOWANCE FOR LOAN LOSSES (Financing Receivable Credit Quality Indicators) (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans $ 3,149,243us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome $ 2,857,095us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome $ 2,930,747us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 2,083,594us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
1,891,904us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
 
Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 1,034,238us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
939,643us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
 
Commercial and Industrial [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 695,267us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
600,704us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
 
Commercial and Industrial [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 695,267us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
600,704us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
 
Real Estate Construction and Development [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 89,851us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
121,662us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
 
Real Estate Construction and Development [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 89,851us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
121,662us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
 
Multi-family [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 115,434us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
121,304us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
 
Multi-family [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 115,434us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
121,304us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
 
Commercial Real Estate [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 1,183,042us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
1,048,234us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
 
Commercial Real Estate [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 1,183,042us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
1,048,234us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
 
Residential Mortgage [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 621,168us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
572,164us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
 
Residential Mortgage [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 621,168us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
572,164us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
 
Home Equity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 395,542us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
349,324us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
 
Home Equity [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 395,542us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
349,324us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
 
Consumer and Installment and Net Deferred Loan Fees [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 17,528us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
18,155us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
 
Consumer and Installment and Net Deferred Loan Fees [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 17,528us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
18,155us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
 
Pass [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 1,976,896us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
1,694,392us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 932,170us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
825,214us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Commercial and Industrial [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 653,951us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
559,243us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Real Estate Construction and Development [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 85,973us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
42,429us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Multi-family [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 89,148us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
91,001us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Commercial Real Estate [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 1,147,824us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
1,001,719us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Residential Mortgage [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 528,388us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
468,642us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Home Equity [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 386,448us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
338,562us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Pass [Member] | Consumer and Installment and Net Deferred Loan Fees [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 17,334us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
18,010us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_PassMember
 
Special mention [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 45,492us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
38,854us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Special mention [Member] | Commercial and Industrial [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 18,713us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
16,211us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Special mention [Member] | Real Estate Construction and Development [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 143us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
929us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Special mention [Member] | Multi-family [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 5,945us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Special mention [Member] | Commercial Real Estate [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 20,691us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
21,714us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Substandard [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 20,083us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
100,671us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 5,096us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
8,833us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Commercial and Industrial [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 12,833us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
14,727us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Real Estate Construction and Development [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
73,390us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Multi-family [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 610us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
555us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Commercial Real Estate [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 6,640us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
11,999us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Residential Mortgage [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 2,662us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
5,306us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Home Equity [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 2,263us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
3,401us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Substandard [Member] | Consumer and Installment and Net Deferred Loan Fees [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 171us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
126us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SubstandardMember
 
Performing troubled debt restructuring [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 4,391us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
32,474us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 76,228us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
78,153us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Commercial and Industrial [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 284us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Real Estate Construction and Development [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 342us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Multi-family [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
27,955us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Commercial Real Estate [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 3,765us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
4,519us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Residential Mortgage [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 76,228us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
78,153us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Home Equity [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Performing troubled debt restructuring [Member] | Consumer and Installment and Net Deferred Loan Fees [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
0us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_PerformingTdrMember
 
Nonaccrual [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 36,732us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
25,513us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 20,744us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
27,443us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Commercial and Industrial [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 9,486us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
10,523us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_CommercialFinancialAndAgriculturalMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Real Estate Construction and Development [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 3,393us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
4,914us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_RealEstateConstructionAndDevelopmentMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Multi-family [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 19,731us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
1,793us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= fbspra_MultiFamilyResidentialMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Commercial Real Estate [Member] | Commercial Loan Portfolio Credit Exposure by Internally Assigned Credit Grade [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 4,122us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
8,283us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstateMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Residential Mortgage [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 13,890us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
20,063us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Home Equity [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans 6,831us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
7,361us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
 
Nonaccrual [Member] | Consumer and Installment and Net Deferred Loan Fees [Member] | Consumer Loan Portfolio Credit Exposure by Payment Activity [Member]      
Financing Receivable, Allowance for Credit Losses [Line Items]      
Total loans $ 23us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember
$ 19us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
/ us-gaap_FinancialInstrumentAxis
= fbspra_OneToFourFamilyResidentialMortgageDivisionPortfolioAndConsumerAndInstallmentPortfolioMember
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
/ us-gaap_InternalCreditAssessmentAxis
= fbspra_NonAccrualMember