Name:	thff_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodTotalFairValue
Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Description of award terms as to how many incremental shares or portion of an award will vest in second year and will no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, shown as a percentage.

No definition available.

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

Description of award terms as to how many incremental shares or portion of an award will vest in third year and will no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, shown as a percentage.

No definition available.

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

Description of award terms as to how many shares or portion of an award will vest in first year and will no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, shown as a percentage.

No definition available.

Name:	thff_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingRightsPercentageForFirstYear
Namespace Prefix:	thff_
Data Type:	dtr-types:percentItemType
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Period Type:	duration

- References

No definition available.

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

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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8672-108599

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

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901