Allowance for Credit Losses - Activity for TDR. (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	$ 4,146	$ 4,460	$ 4,799	$ 4,206
Added/(Disposed)		172	(178)	785
Charged Off				(102)
Payments	(199)	(143)	(674)	(400)
Ending Balance	3,947	4,489	3,947	4,489
Commercial
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	672		407
Added/(Disposed)			305
Payments	(19)		(59)
Ending Balance	653		653
Residential
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	3,474	3,904	3,686	3,589
Added/(Disposed)			128	491
Charged Off				(27)
Payments	(180)	(91)	(520)	(240)
Ending Balance	$ 3,294	3,813	3,294	3,813
Consumer
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance		556	706	617
Added/(Disposed)		172	(611)	294
Charged Off				(75)
Payments		(52)	$ (95)	(160)
Ending Balance		$ 676		$ 676

The amount of the additions (disposals) related to a troubled debt restructuring.

No definition available.

Name:	thff_FinancingReceivableImpairedTroubledDebtRestructuringAdditionsDisposalsDuringPeriod
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Financing Receivable, Impaired, Troubled Debt Restructuring, Payments During Period

+ References

No definition available.

+ Details

Name:	thff_FinancingReceivableImpairedTroubledDebtRestructuringPaymentsDuringPeriod
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ 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 http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: