Fair Value (Details Textual) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other real estate owned	$ 236	$ 108
Impaired Loans [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 10,964	$ 12,839
Fair Value Measurements Unobservable Inputs	Discount rate for age of appraisal and market conditions	Discount rate for age of appraisal and market conditions
Other Real Estate [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 3,531	$ 3,359
Fair Value Measurements Unobservable Inputs	Discount rate	Discount rate
State and Municipal Obligation [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 1,545	$ 1,895
Fair Value Measurements Unobservable Inputs	Discount rate	Discount rate
Measurement Input, Discount Rate [Member] \| Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Real Estate, measurement input	0
Measurement Input, Discount Rate [Member] \| Minimum [Member] \| Impaired Loans [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impaired loans, measurement input	0	0
Measurement Input, Discount Rate [Member] \| Minimum [Member] \| Other Real Estate [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Real Estate, measurement input	0.0500	0.0500
Measurement Input, Discount Rate [Member] \| Minimum [Member] \| State and Municipal Obligation [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
State and Municipal Obligations, measurement input	0.0305	0.0305
Measurement Input, Discount Rate [Member] \| Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Real Estate, measurement input	0.50
Measurement Input, Discount Rate [Member] \| Maximum [Member] \| Impaired Loans [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Impaired loans, measurement input	0.5000	0.5000
Measurement Input, Discount Rate [Member] \| Maximum [Member] \| Other Real Estate [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other Real Estate, measurement input	0.2000	0.2000
Measurement Input, Discount Rate [Member] \| Maximum [Member] \| State and Municipal Obligation [Member] \| Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
State and Municipal Obligations, measurement input	0.0550	0.0550

The description of the unobservable input used to measure the fair value of assets at the end of the period.

No definition available.

Name:	thff_FairValueMeasurementsUnobservableInputs
Namespace Prefix:	thff_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

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

- Definition

Value of input used to measure debt instrument, including, but not limited to, convertible and non-convertible debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

Name:	us-gaap_DebtInstrumentMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Value of input used to measure loan, held for sale.

+ References

+ Details

Name:	us-gaap_LoansHeldForSaleMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(4))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

+ Details

Name:	us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=thff_ImpairedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=thff_OtherRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=thff_StateAndMunicipalObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: