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Allocation of Allowance for Loan Losses (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]    
Allowance for loan losses $ 5,928us-gaap_LoansAndLeasesReceivableAllowance $ 6,095us-gaap_LoansAndLeasesReceivableAllowance
Allowance for loan losses, percentage of total 100.00%fbms_AllowanceForCreditLossesPercentage 100.00%fbms_AllowanceForCreditLossesPercentage
Commercial Non Real Estate    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Allowance for loan losses 735us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_CommercialNonRealEstateMember		 713us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_CommercialNonRealEstateMember
Allowance for loan losses, percentage of total 15.40%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_CommercialNonRealEstateMember		 15.30%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_CommercialNonRealEstateMember
Commercial Real Estate    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Allowance for loan losses 2,715us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember		 3,355us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember
Allowance for loan losses, percentage of total 58.20%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember		 57.90%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_CommercialRealEstateMember
Consumer Real Estate    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Allowance for loan losses 1,487us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_ConsumerRealEstateMember		 1,852us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_ConsumerRealEstateMember
Allowance for loan losses, percentage of total 23.70%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_ConsumerRealEstateMember		 24.20%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_ConsumerRealEstateMember
Consumer    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Allowance for loan losses 167us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ConsumerLoanMember		 175us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ConsumerLoanMember
Allowance for loan losses, percentage of total 2.70%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ConsumerLoanMember		 2.60%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= us-gaap_ConsumerLoanMember
Unallocated    
Financing Receivable, Allowance for Credit Losses [Line Items]    
Allowance for loan losses $ 824us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_UnallocatedAllowanceMember		 $ 0us-gaap_LoansAndLeasesReceivableAllowance
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_UnallocatedAllowanceMember
Allowance for loan losses, percentage of total 0.00%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_UnallocatedAllowanceMember		 0.00%fbms_AllowanceForCreditLossesPercentage
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= fbms_UnallocatedAllowanceMember