Dividends from Subsidiaries - Additional Information (Detail)	Dec. 31, 2018 USD ($)
Equity [Abstract]
Amount available for the declaration of dividends by subsidiary banks without prior approval of regulatory agencies	$ 233,956,000

Amount available for dividend distribution without approval from regulatory agencies.

No definition available.

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