Cash Flow Information - Supplemental Information on Cash Flows and Noncash Transactions (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental cash flow information:
Interest paid	$ 4,231us-gaap_InterestPaid	$ 4,077us-gaap_InterestPaid	$ 5,419us-gaap_InterestPaid
Federal income taxes paid	28,711us-gaap_IncomeTaxesPaid	21,790us-gaap_IncomeTaxesPaid	20,779us-gaap_IncomeTaxesPaid
Schedule of noncash investing and financing activities:
Assets acquired through foreclosure	1,385ffin_TransferOfLoansToForeclosedAssets	1,615ffin_TransferOfLoansToForeclosedAssets	2,459ffin_TransferOfLoansToForeclosedAssets
Investment securities purchased but not settled	$ 1,248	$ 7,248	$ 8,589

Investment securities purchased as of the balance sheet date that have not been settled with the seller.

No definition available.

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Namespace Prefix:	ffin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Real estate and other loan collateral acquired through foreclosure.

+ References

No definition available.

+ Details

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Namespace Prefix:	ffin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

Name:	us-gaap_IncomeTaxesPaid
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of cash paid for interest during the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

Name:	us-gaap_InterestPaid
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_NoncashInvestingAndFinancingItemsAbstract
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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Balance Type:	na
Period Type:	duration