Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 142,233us-gaap_CashAndDueFromBanks	$ 190,387us-gaap_CashAndDueFromBanks	$ 160,469us-gaap_CashAndDueFromBanks
FEDERAL FUNDS SOLD	5,460us-gaap_FederalFundsSold	8,760us-gaap_FederalFundsSold	3,620us-gaap_FederalFundsSold
INTEREST-BEARING DEPOSITS IN BANKS	18,275us-gaap_InterestBearingDepositsInBanks	54,324us-gaap_InterestBearingDepositsInBanks	3,772us-gaap_InterestBearingDepositsInBanks
INTEREST-BEARING TIME DEPOSITS IN BANKS	9,170ffin_InterestBearingTimeDepositsInBanks	17,002ffin_InterestBearingTimeDepositsInBanks	30,026ffin_InterestBearingTimeDepositsInBanks
Available-for-sale securities	2,689,330us-gaap_AvailableForSaleSecurities	2,415,856us-gaap_AvailableForSaleSecurities	2,163,017us-gaap_AvailableForSaleSecurities
Held-to-maturity securities	310us-gaap_HeldToMaturitySecurities	441us-gaap_HeldToMaturitySecurities	582us-gaap_HeldToMaturitySecurities
Loans	2,900,879us-gaap_LoansReceivableNet	2,901,167us-gaap_LoansReceivableNet	2,664,024us-gaap_LoansReceivableNet
SHORT-TERM BORROWINGS	401,898us-gaap_ShortTermBorrowings	367,110us-gaap_ShortTermBorrowings	383,220us-gaap_ShortTermBorrowings
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	142,233	190,387
FEDERAL FUNDS SOLD	5,460	8,760
INTEREST-BEARING DEPOSITS IN BANKS	18,275	54,324
Deposits with no stated maturities	4,213,899	4,101,263
Carrying Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	9,170	17,002
Held-to-maturity securities	310	441
Accrued interest receivable	25,238	28,998
Deposits with stated maturities	623,108	648,992
SHORT-TERM BORROWINGS	401,898	367,110
Accrued interest payable	235	237
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,689,330	2,415,856
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	2,900,879	2,901,167
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	142,233	190,387
FEDERAL FUNDS SOLD	5,460	8,760
INTEREST-BEARING DEPOSITS IN BANKS	18,275	54,324
Deposits with no stated maturities	4,213,899	4,101,263
Estimated Fair Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	9,185	17,032
Held-to-maturity securities	317	447
Accrued interest receivable	25,238	28,998
Deposits with stated maturities	624,926	650,893
SHORT-TERM BORROWINGS	401,898	367,110
Accrued interest payable	235	237
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,689,330	2,415,856
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 2,907,114	$ 2,906,399

Accrued interest payable.

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Deposits with no stated maturities.

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Deposits with stated maturities.

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Interest-bearing time deposits in banks.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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For banks and other depository institutions: Includes cash on hand (currency and coin), cash items in process of collection, noninterest bearing deposits due from other financial institutions (including corporate credit unions), and noninterest bearing balances with the Federal Reserve Banks, Federal Home Loan Banks and central banks.

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The amount outstanding of funds lent to other depository institutions, securities brokers, or securities dealers in the form of Federal Funds sold; for example, immediately available funds lent under agreements or contracts that mature in one business day or roll over under a continuing contract, regardless of the nature of the transaction or the collateral involved, excluding overnight lending for commercial and industrial purposes. Also include Federal Funds sold under agreements to resell on a gross basis, excluding (1) sales of term Federal Funds, (2) due bills representing purchases of securities or other assets by the reporting bank that have not yet been delivered and similar instruments, (3) resale agreements that mature in more than one business day involving assets other than securities, and (4) yield maintenance dollar repurchase agreements.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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For banks and other depository institutions (including Federal Reserve Banks, if applicable): Interest-bearing deposits in other financial institutions for relatively short periods of time including, for example, certificates of deposits, which are presented separately from cash on the balance sheet.

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Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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