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Loans and Allowance for Loan Losses - Schedule of How Loans Were Modified as Troubled Debt Restructured Loans (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Financing Receivable, Modifications [Line Items]    
Adjusted Interest Rate $ 0ffin_AdjustedInterestRate $ 0ffin_AdjustedInterestRate
Extended Maturity 128ffin_ExtendedMaturity 181ffin_ExtendedMaturity
Combined Rate and Maturity 24ffin_CombinedRateAndMaturity 3ffin_CombinedRateAndMaturity
Commercial [Member]    
Financing Receivable, Modifications [Line Items]    
Adjusted Interest Rate 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Extended Maturity 128ffin_ExtendedMaturity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember		 97ffin_ExtendedMaturity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember
Agriculture [Member]    
Financing Receivable, Modifications [Line Items]    
Adjusted Interest Rate 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= ffin_AgriculturalMember		 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= ffin_AgriculturalMember
Real Estate [Member]    
Financing Receivable, Modifications [Line Items]    
Adjusted Interest Rate 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_RealEstateMember		 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_RealEstateMember
Extended Maturity   76ffin_ExtendedMaturity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_RealEstateMember
Consumer [Member]    
Financing Receivable, Modifications [Line Items]    
Adjusted Interest Rate 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember		 0ffin_AdjustedInterestRate
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
Extended Maturity   8ffin_ExtendedMaturity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
Combined Rate and Maturity $ 24ffin_CombinedRateAndMaturity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember		 $ 3ffin_CombinedRateAndMaturity
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember