Amount of allowance for credit loss excluding accrued on unfunded.

No definition available.

Name:	thff_AllowanceForCreditLossOnUnfundedCommitment
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all compensation arrangements payable. Represents currently earned compensation under compensation arrangements that are not actually paid until a later date.

+ References

No definition available.

+ Details

Name:	thff_CompensationLiability
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the amount of Deferred compensation liability outstanding as of the reporting period.

+ References

No definition available.

+ Details

Name:	thff_DeferredCompensationLiabilityIncentivePlan
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of expenditure on deferred compensation.

+ References

No definition available.

+ Details

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

- Definition

Incentive Award, Payment Period

+ References

No definition available.

+ Details

Name:	thff_IncentiveAwardPaymentPeriod
Namespace Prefix:	thff_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Income Tax Examination Likelihood Of Unfavorable Settlement, Percent

+ References

No definition available.

+ Details

Name:	thff_IncomeTaxExaminationLikelihoodOfUnfavorableSettlementPercent
Namespace Prefix:	thff_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of loan production offices.

+ References

No definition available.

+ Details

Name:	thff_NumberOfLoanProductionOffices
Namespace Prefix:	thff_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Branches Entity Operates

+ References

No definition available.

+ Details

Name:	thff_NumberofBranchesEntityOperates
Namespace Prefix:	thff_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Investment Subsidiaries

+ References

No definition available.

+ Details

Name:	thff_NumberofInvestmentSubsidiaries
Namespace Prefix:	thff_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The repayments term of deferred fee.

+ References

No definition available.

+ Details

Name:	thff_RepaymentOfDeferredFeesAndInterestInYears
Namespace Prefix:	thff_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which payment is to be made of the short term portion of share based compensation after vesting period, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

No definition available.

+ Details

Name:	thff_ShareBasedCompensationLongTermPortionPaymentPeriod
Namespace Prefix:	thff_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Period over which payment is to be made of the short term portion of share based compensation after year end, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

No definition available.

+ Details

Name:	thff_ShareBasedCompensationShortTermPortionPaymentPeriod
Namespace Prefix:	thff_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479830/718-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

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

- Definition

Aggregate carrying value as of the balance sheet date of the liabilities for all deferred compensation arrangements payable within one year (or the operating cycle, if longer). Represents currently earned compensation under compensation arrangements that is not actually paid until a later date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 710
-SubTopic 10
-Name Accounting Standards Codification
-Section 25
-Paragraph 9
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483070/710-10-25-9

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 710
-SubTopic 10
-Section 30
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483043/710-10-30-2

+ Details

Name:	us-gaap_DeferredCompensationLiabilityCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 24
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482548/350-20-55-24

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482573/350-20-50-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482573/350-20-50-2

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482598/350-20-45-2

+ Details

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

- Definition

Amount of investment in equity method investee and investment in and advance to affiliate.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(18))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

+ Details

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

- Definition

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-18

+ Details

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

- Definition

Amount of expense for salary and wage arising from service rendered by officer. Excludes allocated cost, labor-related nonsalary expense, and direct and overhead labor cost included in cost of good and service sold.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(4))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

+ Details

Name:	us-gaap_OfficersCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_PropertyPlantAndEquipmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 924
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479941/924-10-S99-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 30
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-30

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 42
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-42

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 40
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-40

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 22
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-22

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 32
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-32

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Topic 280
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 41
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482810/280-10-50-41

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479806/606-10-50-4

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480429/718-10-50-2

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Value transferred from mortgage loans to real estate owned (REO) in noncash transactions.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-4

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-3

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-5

+ Details

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

- 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:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_FurnitureAndFixturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingAndBuildingImprovementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_VigoCountyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_DaviessCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ClayCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_GreeneCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_KnoxCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ParkeCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_PutnamCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_SullivanCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_VanderburghCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_AllenCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_MonroeCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_VermillionCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ChampaignCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ClarkCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ColesCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_CrawfordCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_FranklinCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_JasperCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_LawrenceCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_LivingstonCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_MarionCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_McLeanCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_RichlandCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_VermilionCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_WayneCountyIllinoisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_CallowayCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_BreckinridgeCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ChristianCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_FultonCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_HancockCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_HopkinsCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_MarshallCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_ToddCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_TriggCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_WarrenCountyKentuckyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_CheathamCountyTennesseeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_MontgomeryCountyTennesseeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_HamiltonCountyIndianaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_RutherfordCountyTennesseeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=thff_WilliamsonCountyTennesseeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=thff_FirstFinancialBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=thff_ServicingFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

BUSINESS AND SIGNIFICANT ACCOUNTING POLICIES (Details)	1 Months Ended	12 Months Ended
BUSINESS AND SIGNIFICANT ACCOUNTING POLICIES (Details)	Oct. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) item segment subsidiary	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Property, Plant and Equipment
Number of investment subsidiaries (subsidiary) \| subsidiary		2
Number of branches entity operates (branches)		70
Number of loan production offices		6
Transfer to other real estate \| $		$ 88,000	$ 570,000	$ 43,000
Impairment of goodwill \| $	$ 0
Repayment of deferred fees and interest in years		10 years
Expenses for deferred compensation \| $		$ 49,000	78,000	117,000
Allowance for credit losses on unfunded commitments \| $		2,000,000.0	2,100,000
Compensation expense \| $		0	0	0
Deferred compensation liability, current \| $		$ 1,100,000	1,200,000
Incentive award, payment period		15 years
Compensation liability \| $		$ 3,800,000	4,800,000
Short term portion, payment period		75 days
Vesting period		3 years
Long term portion, payment period		75 days
Officers' compensation expense \| $		$ 2,900,000	2,000,000.0	2,300,000
Deferred compensation liability incentive plan \| $		$ 1,800,000	1,600,000
Likelihood of unfavorable settlement (percent)		50.00%
Number of reportable segments \| segment		1
Minimum
Property, Plant and Equipment
Finite-lived intangible asset, useful life		10 years
Maximum
Property, Plant and Equipment
Finite-lived intangible asset, useful life		12 years
Furniture and Fixtures \| Minimum
Property, Plant and Equipment
Property, plant and equipment, estimated useful lives		3 years
Furniture and Fixtures \| Maximum
Property, Plant and Equipment
Property, plant and equipment, estimated useful lives		5 years
Buildings And Leasehold Improvements \| Minimum
Property, Plant and Equipment
Property, plant and equipment, estimated useful lives		33 years
Buildings And Leasehold Improvements \| Maximum
Property, Plant and Equipment
Property, plant and equipment, estimated useful lives		39 years
Vigo County
Property, Plant and Equipment
Number of branches entity operates (branches)		7
Daviess County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Clay County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		3
Greene County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Knox County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Parke County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Putnam County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Sullivan County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Vanderburgh County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Allen County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Monroe County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Vermillion County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		3
Champaign County, Illinois
Property, Plant and Equipment
Number of loan production offices		4
Clark County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Coles County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Crawford County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Franklin County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Jasper County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Lawrence County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Livingston County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Marion County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		2
McLean County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Richland County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Vermilion County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		5
Wayne County, Illinois
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Calloway County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Breckinridge County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Christian County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		3
Fulton County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Hancock County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Hopkins County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Marshall County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		2
Todd County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Trigg County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Warren County, Kentucky
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Cheatham County, Tennessee
Property, Plant and Equipment
Number of branches entity operates (branches)		3
Montgomery County, Tennessee
Property, Plant and Equipment
Number of branches entity operates (branches)		3
Hamilton County, Indiana
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Rutherford County, Tennessee
Property, Plant and Equipment
Number of branches entity operates (branches)		1
Williamson County, Tennessee
Property, Plant and Equipment
Number of branches entity operates (branches)		1
First Financial Bank
Property, Plant and Equipment
Investments in subsidiaries \| $		$ 1,000,000,000.0
Service fees
Property, Plant and Equipment
Revenue within scope of ASC 606 \| $		$ 1,300,000	$ 1,400,000	$ 1,300,000