Securities - Roll forward of the credit losses recognized in earnings (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 2,974	$ 2,974
Reductions for securities called during the period	(299)	0
Ending balance	$ 2,675	$ 2,974

This item represents reductions in the cumulative amount of credit losses recognized in earnings for securities called during the period.

No definition available.

Name:	thff_InvestmentSecuritiesCreditLossesRecognizedInEarningsReductionsForSecuritiesCalled
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration