Allowance for Loan Losses (Details 4) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	$ 4,542	$ 4,464	$ 4,799	$ 4,206
Added	(273)	187	(178)	613
Charged Off	0	(59)	0	(102)
Payments	(123)	(132)	(475)	(257)
Ending Balance	4,146	4,460	4,146	4,460
Commercial Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	407	0	407	0
Added	305	0	305	0
Charged Off	0	0	0	0
Payments	(40)	0	(40)	0
Ending Balance	672	0	672	0
Residential Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	3,445	3,888	3,686	3,589
Added	101	113	128	491
Charged Off	0	(27)	0	(27)
Payments	(72)	(70)	(340)	(149)
Ending Balance	3,474	3,904	3,474	3,904
Consumer Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	690	576	706	617
Added	(679)	74	(611)	122
Charged Off	0	(32)	0	(75)
Payments	(11)	(62)	(95)	(108)
Ending Balance	$ 0	$ 556	$ 0	$ 556

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

No definition available.

Name:	thff_FinancingReceivableImpairedTroubledDebtRestructuringAdditionsDuringPeriod
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: