Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024
Available-for-sale investment securities:
Estimated Fair Value	$ 4,886,548,000	$ 4,617,759,000	$ 4,573,024,000
Loans held for sale	33,233,000	8,235,000	19,668,000
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	153,835,000	273,562,000	341,976,000
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,521,364,000	1,440,245,000	1,445,373,000
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,411,874,000	2,350,534,000	2,316,085,000
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	707,018,000	447,752,000	350,022,000
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	955,000	238,000	518,000
Forward mortgage-backed securities trades	0	0	0
Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	84,000	0
Forward mortgage-backed securities trades	(584,000)	0	(4,000)
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,886,548,000	4,617,759,000	4,573,024,000
Loans held for sale	31,611,000	7,793,000	19,393,000
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	153,835,000	273,562,000	341,976,000
Fair Value, Measurements, Recurring [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,521,364,000	1,440,245,000	1,445,373,000
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	87,949,000	101,238,000	99,627,000
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,411,874,000	2,350,534,000	2,316,085,000
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	707,018,000	447,752,000	350,022,000
Fair Value, Measurements, Recurring [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	955,000	238,000	518,000
Fair Value, Measurements, Recurring [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	584,000	84,000	(4,000)
Fair Value, Measurements, Recurring [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,508,000	4,428,000	19,941,000
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	158,343,000	277,990,000	361,917,000
Loans held for sale	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	153,835,000	273,562,000	341,976,000
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,508,000	4,428,000	19,941,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,728,205,000	4,339,769,000	4,211,107,000
Loans held for sale	31,611,000	7,793,000	19,393,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,521,364,000	1,440,245,000	1,445,373,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	87,949,000	101,238,000	99,627,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,411,874,000	2,350,534,000	2,316,085,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	707,018,000	447,752,000	350,022,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	584,000	84,000	(4,000)
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Loans held for sale	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	955,000	238,000	518,000
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	$ 0	$ 0	$ 0

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/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 103
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-103

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (bbb)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (aa)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481800/320-10-50-2

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481830/320-10-45-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479130/326-30-45-1

+ 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
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4B
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4B

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 22
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-22

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483466/210-20-50-3

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-7

Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-SubTopic 20
-Topic 860
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481326/860-20-50-4D

+ 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

+ Details

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

- Definition

Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 948
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477011/948-310-45-1

+ Details

Name:	us-gaap_LoansReceivableHeldForSaleNetNotPartOfDisposalGroup
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_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_FinancialInstrumentAxis=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:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: