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Fair Value Disclosures - Schedule of Gain Loss on Sale of Mortgage Loans (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Schedule Of Gain Loss On Sale Of Mortgage Loans [Abstract]
Realized gain on sale and fees on mortgage loans	[1]	$ 4,537	$ 3,463	$ 10,678	$ 9,813
Change in fair value on loans held-for-sale and IRLCs		(531)	(10)	954	171
Change in forward mortgage-backed securities trades		369	(94)	(299)	190
Total gain on sale of mortgage loans		$ 4,375	$ 3,359	$ 11,333	$ 10,174
[1] This includes gains on loans held-for-sale carried under the fair value method and lower of cost or market.

X

- Definition

Change in fair value on loans held for sale and Irlcs.

+ References

No definition available.

+ Details

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X

- Definition

Change in forward mortgage-backed securities trades.

+ References

No definition available.

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- Definition

Realized gain on sale and fees on mortgage loans.

+ References

No definition available.

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Namespace Prefix:	ffin_
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- Definition

Schedule of gain loss on sale of mortgage loans.

+ References

No definition available.

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- Definition

The gains (losses) included in earnings that represent the difference between the sale price and the carrying value of loans made to finance real estate acquisitions. This element refers to the gain (loss) and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-SubTopic 310
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477409/944-310-45-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-SubTopic 310
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477409/944-310-45-2

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-SubTopic 310
-Topic 944
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477363/944-310-50-1

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