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Loans Held for Sale - Additional Information (Detail) - USD ($)	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Loans Receivables Held For Sale Net [Abstract]
Loans held for sale	$ 26,015,000	$ 8,235,000	$ 20,114,000
Loans held-for-sale at the lower of cost or fair value	$ 835,000	$ 442,000	$ 3,601,000

X

- Definition

Loans held for sale valued at the lower of cost or fair value

+ References

No definition available.

+ Details

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

X

- Definition

Loans Receivables Held For Sale Net [Abstract].

+ References

No definition available.

+ Details

Name:	ffin_LoansReceivablesHeldForSaleNetAbstract
Namespace Prefix:	ffin_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- 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

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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