Fair Value Disclosures - Schedule of Estimated Fair Values and Carrying Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	$ 190,387us-gaap_CashAndDueFromBanks	$ 183,084us-gaap_CashAndDueFromBanks
FEDERAL FUNDS SOLD	8,760us-gaap_FederalFundsSold	3,430us-gaap_FederalFundsSold
INTEREST-BEARING DEPOSITS IN BANKS	54,324us-gaap_InterestBearingDepositsInBanks	25,498us-gaap_InterestBearingDepositsInBanks
INTEREST-BEARING TIME DEPOSITS IN BANKS	17,002ffin_InterestBearingTimeDepositsInBanks	31,917ffin_InterestBearingTimeDepositsInBanks
Available-for-sale securities	2,415,856us-gaap_AvailableForSaleSecurities	2,057,723us-gaap_AvailableForSaleSecurities
Held-to-maturity securities	441us-gaap_HeldToMaturitySecurities	684us-gaap_HeldToMaturitySecurities
Loans	2,901,167us-gaap_LoansReceivableNet	2,655,548us-gaap_LoansReceivableNet
SHORT-TERM BORROWINGS	367,110us-gaap_ShortTermBorrowings	463,888us-gaap_ShortTermBorrowings
Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	64,668
Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,351,188
Carrying Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	190,387	183,084
FEDERAL FUNDS SOLD	8,760	3,430
INTEREST-BEARING DEPOSITS IN BANKS	54,324	25,498
Deposits with no stated maturities	4,101,263	3,448,449
Carrying Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	17,002	31,917
Held-to-maturity securities	441	684
Accrued interest receivable	28,998	26,865
Deposits with stated maturities	648,992	686,626
SHORT-TERM BORROWINGS	367,110	463,888
Accrued interest payable	237	299
Carrying Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,415,856	2,057,723
Carrying Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	2,901,167	2,655,548
Estimated Fair Value [Member] \| Level 1 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
CASH AND DUE FROM BANKS	190,387	183,084
FEDERAL FUNDS SOLD	8,760	3,430
INTEREST-BEARING DEPOSITS IN BANKS	54,324	25,498
Deposits with no stated maturities	4,101,263	3,448,449
Estimated Fair Value [Member] \| Level 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	17,032	32,059
Held-to-maturity securities	447	694
Accrued interest receivable	28,998	26,865
Deposits with stated maturities	650,893	688,876
SHORT-TERM BORROWINGS	367,110	463,888
Accrued interest payable	237	299
Estimated Fair Value [Member] \| Levels 1 and 2 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale securities	2,415,856	2,057,723
Estimated Fair Value [Member] \| Level 3 Inputs [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 2,906,399	$ 2,667,743

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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- Definition

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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