Allowance for Credit Losses - Additional Information (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2023 USD ($) loan	Dec. 31, 2022 loan
Allowance for loan losses
Number of modifications resulted in the permanent reduction	0	0
Number of loans related to TDR	0
Minimum outstanding balance of non-homogeneous loans to be individually evaluated as to credit risk \| $	$ 100

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.

+ Details

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

- Definition

Number of modifications resulted in permanent reduction in investment in the loan.

+ References

No definition available.

+ Details

Name:	thff_NumberOfModificationsResultedInPermanentReductionInInvestment
Namespace Prefix:	thff_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI https://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration