Allowance for Loan Losses (Details 4) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	$ 4,460	$ 3,899	$ 4,206	$ 4,194
Added	172	394	785	652
Charged Off	0	(30)	(102)	(86)
Payments	(143)	(166)	(400)	(663)
Ending Balance	4,489	4,097	4,489	4,097
Commercial Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	0	0	0	11
Added	0	0	0	0
Charged Off	0	0	0	0
Payments	0	0	0	(11)
Ending Balance	0	0	0	0
Residential Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	3,904	3,231	3,589	3,485
Added	0	313	491	436
Charged Off	0	0	(27)	(6)
Payments	(91)	(112)	(240)	(483)
Ending Balance	3,813	3,432	3,813	3,432
Consumer Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring [Line Items]
Beginning Balance	556	668	617	698
Added	172	81	294	216
Charged Off	0	(30)	(75)	(80)
Payments	(52)	(54)	(160)	(169)
Ending Balance	$ 676	$ 665	$ 676	$ 665

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: