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Securities - Roll forward of the credit losses recognized in earnings (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Credit Losses Recognized in Earnings [Roll Forward] |  |  |  |
Beginning balance | $ 2,974 | $ 2,974 | $ 2,974 | $ 2,974
Reductions for securities called during the period | 0 | 0 | 0 | 0
Ending balance | $ 2,974 | $ 2,974 | $ 2,974 | $ 2,974

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- Definition A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | thff_CreditLossesRecognizedInEarningsRollForward
Namespace Prefix: | thff_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The cumulative amount of credit losses recognized in earnings related to investment securities held. + References No definition available. + Details
Name: | thff_InvestmentSecuritiesCreditLossesRecognizedInEarnings
Namespace Prefix: | thff_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for securities called during the period. + References No definition available. + Details
Name: | thff_InvestmentSecuritiesCreditLossesRecognizedInEarningsReductionsForSecuritiesCalled
Namespace Prefix: | thff_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration