ALLOWANCE FOR CREDIT LOSSES- Additional Information (Details) $ in Thousands	12 Months Ended
Dec. 31, 2023 USD ($) loan
ALLOWANCE FOR LOAN LOSSES
Number of loans modified that had a payment default \| loan	0
Minimum outstanding balance of non-homogeneous loans to be individually evaluated as to credit risk \| $	$ 100
Nonperforming loans past due	90 days

Represents the minimum outstanding balance of loans to be individually reviewed and classified as to credit risk. Homogeneous loans (at all levels of outstanding balances) are not reviewed individually, but rather as part of a group.

No definition available.

Name:	thff_LoansAndLeasesReceivableMinimumOutstandingBalanceToBeIndividuallyEvaluated
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant