No definition available.

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiability
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after allowance for bad debt, of mortgage loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansReceivableHeldForSaleNetNotPartOfDisposalGroupMortgage
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=ffin_ForwardMortgageBackedSecuritiesTradesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=us-gaap_CollateralizedSecuritiesOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 3,158,777	$ 3,158,777	$ 3,087,473
Loans held-for-sale		21,672	15,130
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	9,962
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	301		60,330
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,257,871		1,420,850
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,454,545		1,219,097
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 431,300		375,737
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value		765	500
Forward Mortgage-Backed Securities Traded [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities traded		403
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		3,158,777	3,087,473
Loans held-for-sale		19,185
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		450	7,031
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		9,962
Fair Value, Measurements, Recurring [Member] \| Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		301	60,330
Fair Value, Measurements, Recurring [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		1,257,871	1,420,850
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		1,454,545	1,219,097
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		431,300	375,737
Fair Value, Measurements, Recurring [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value		765	500
Fair Value, Measurements, Recurring [Member] \| Forward Mortgage-Backed Securities Traded [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities traded		403
Fair Value, Measurements, Recurring [Member] \| Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		4,348	4,428
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		14,310	4,428
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		9,962
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Other Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		4,348	4,428
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		3,144,467	3,083,045
Loans held-for-sale		19,185
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		450	7,031
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		301	60,330
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		1,257,871	1,420,850
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		1,454,545	1,219,097
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value		431,300	375,737
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value		765	$ 500
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Forward Mortgage-Backed Securities Traded [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities traded		$ 403