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v3.22.1

Fair Value (Details 2) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2022 | Dec. 31, 2021
State and Municipal Obligations |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value | $ 1,545 | $ 1,895
Unobservable Inputs | Discount rate | Discount rate
Impaired Loans |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value | $ 10,964 | $ 12,839
Unobservable Inputs | Discount rate for age of appraisal and market conditions | Discount rate for age of appraisal and market conditions

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- Definition The description of the unobservable input used to measure the fair value of assets at the end of the period. + References No definition available. + Details
Name: | thff_FairValueMeasurementsUnobservableInputs
Namespace Prefix: | thff_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- 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

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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 No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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