Fair Value Disclosures - Other Real Estate Owned (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Carrying value of other real estate owned prior to re-measurement	$ 937	$ 1,025
Write-downs included in gain (loss) on sale of other real estate owned	(288)	(296)
Other real estate owned, fair value	$ 649	$ 729

Write downs included in gain loss on sale of other real estate owned.

No definition available.

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Namespace Prefix:	ffin_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration