Business acquisition special dividends payable by acquiree.

No definition available.

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

- Definition

Loans receivable contractual amount.

+ References

No definition available.

+ Details

Name:	ffin_LoansReceivableContractualAmount
Namespace Prefix:	ffin_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Date when the business acquisition agreement was executed, in YYYY-MM-DD format.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionDateOfAcquisitionAgreement1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares of equity interests issued or issuable to acquire entity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)(4)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

+ Details

Name:	us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ffin_CommercialBancsharesIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=ffin_TbTBancsharesIncorporationAndTheBankTrustOfBryanCollegeStationTexasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Acquisition - Additional Information (Detail) - USD ($)			9 Months Ended
Acquisition - Additional Information (Detail) - USD ($)	Jan. 02, 2020	Sep. 19, 2019	Sep. 30, 2021
Commercial Bancshares, Inc [Member]
Business Acquisition [Line Items]
Business acquisition, agreement date			Sep. 19, 2019
Payment for all outstanding shares of acquired entity by shares	6,275,574
Business acquisition, special dividend	$ 1,920,000
TB&T Bancshares, Inc. and The Bank & Trust of Bryan College Station, Texas [Member]
Business Acquisition [Line Items]
Payment for all outstanding shares of acquired entity by shares		6,275,574
Fair value of total loans	447,702,000		$ 447,702,000
Total loans of contractual amounts	$ 455,181,000