Acquisitions - Financial Assets With Credit Deterioration (Details) - SimplyBank $ in Millions	Jul. 01, 2024 USD ($)
Acquisitions
Fair value of purchased financial assets with credit deterioration	$ 1.7
Gross amount contractual amounts of receivable with credit deterioration	4.7
Contractual cash flow will not be collected	$ 3.0

The amount of contractual cash flow specific to purchased assets with credit deterioration that the entity estimates will not be collected.

No definition available.

+ Details

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

- Definition

The fair value of financial assets with credit deterioration purchased in a business combination on the date of acquisition.

+ References

No definition available.

+ Details

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Namespace Prefix:	thff_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 805
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479907/805-20-50-5

+ Details

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

- Definition

Amount at purchase price of financing receivable purchased with credit deterioration.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 19
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-19

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=thff_SimplybankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: