Acquisition - Schedule of Preliminary Estimated Fair Value Amounts Assigned to Major Asset and Liability Categories at Acquisition Date (Detail) (Orange Savings Bank [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Orange Savings Bank [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	$ 13,494
Securities available-for-sale	107,735
Loans	293,288
Identifiable intangible assets	2,300
Other assets	12,569
Total identifiable assets acquired	429,386
Deposits	385,950
Other liabilities	4,154
Total liabilities assumed	390,104
Fair value of net identifiable assets acquired	39,282
Goodwill resulting from acquisition	$ 23,018

Acquisition - Schedule of Preliminary Estimated Fair Value Amounts Assigned to Major Asset and Liability Categories at Acquisition Date (Detail) (Orange Savings Bank [Member], USD $)
In Thousands, unless otherwise specified

Dec. 31, 2014

Orange Savings Bank [Member]

Cash and cash equivalents

$ 13,494us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Securities available-for-sale

107,735us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsMarketableSecurities
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Loans

293,288us-gaap_LoansReceivableFairValueDisclosure
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Identifiable intangible assets

2,300us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Other assets

12,569ffin_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssets
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Total identifiable assets acquired

429,386us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Deposits

385,950ffin_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDepositLiabilities
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Other liabilities

4,154us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Total liabilities assumed

390,104ffin_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilitiesAndShareholdersEquity
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Fair value of net identifiable assets acquired

39,282us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

Goodwill resulting from acquisition

$ 23,018us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= ffin_OrangeSavingsBankMember

- Definition

Business combination recognized identifiable assets acquired and liabilities assumed deposit liabilities.

+ References

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- Definition

Business combination recognized identifiable assets acquired and liabilities assumed liabilities and shareholders equity.

+ References

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- Definition

Business combination recognized identifiable assets acquired and liabilities assumed other assets.

+ References

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- Definition

Amount of assets acquired at the acquisition date.

+ References

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- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

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- Definition

Amount of investments in debt and equity securities, including, but not limited to, held-to-maturity, trading and available-for-sale expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer, acquired at the acquisition date.

+ References

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- Definition

Amount of other liabilities due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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- Definition

Amount of assets, excluding financial assets and goodwill, that lack physical substance, having a projected indefinite period of benefit, acquired at the acquisition date.

+ References

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- Definition

Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6910749&loc=d3e4845-128472

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-SubTopic 10

-Section 55

-Paragraph 37

-URI http://asc.fasb.org/extlink&oid=25498275&loc=d3e2207-128464

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- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=14024403&loc=d3e13816-109267

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-SubTopic 20

-Section 45

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-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Glossary Loan

-URI http://asc.fasb.org/extlink&oid=6517073

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