Fair Value Disclosures - Other Real Estate Owned (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Carrying value of other real estate owned prior to re-measurement	$ 0	$ 0
Write-downs included in gain (loss) on sale of other real estate owned	0	0
Other real estate owned, fair value	$ 0	$ 0

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

No definition available.

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