Loans and Allowance for Loan Losses - Schedule of Loans Modified and Considered Troubled Debt Restructurings (Detail) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2015 USD ($) SecurityLoan	Sep. 30, 2016 USD ($) SecurityLoan	Sep. 30, 2015 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	7	4	24	13
Pre-Modification Recorded Investment	$ 980	$ 247	$ 4,487	$ 556
Post-Modification Recorded Investment	$ 980	$ 247	$ 4,487	$ 556
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	3	1	14	3
Pre-Modification Recorded Investment	$ 230	$ 66	$ 3,156	$ 139
Post-Modification Recorded Investment	$ 230	$ 66	$ 3,156	$ 139
Agriculture [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan				3
Pre-Modification Recorded Investment				$ 129
Post-Modification Recorded Investment				$ 129
Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	3	1	5	2
Pre-Modification Recorded Investment	$ 706	$ 149	$ 1,169	$ 228
Post-Modification Recorded Investment	$ 706	$ 149	$ 1,169	$ 228
Consumer [Member]
Financing Receivable, Modifications [Line Items]
Number \| SecurityLoan	1	2	5	5
Pre-Modification Recorded Investment	$ 44	$ 32	$ 162	$ 60
Post-Modification Recorded Investment	$ 44	$ 32	$ 162	$ 60

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ffin_AgriculturalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: