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Fair Value (Details 2) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Impaired Loans |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value | $ 120 | $ 1,639
Unobservable Inputs | Discount rate for age of appraisal and market conditions | Discount rate for age of appraisal and market conditions
Other Real Estate |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value | $ 3,717 | $ 603
Unobservable Inputs | Discount rate for age of appraisal and market conditions | Discount rate for age of appraisal and market conditions
State and Municipal Obligations |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31) |  |
Fair Value | $ 2,565 | $ 3,135
Unobservable Inputs | Discount rate Probability of default | Discount rate Probability of default

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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=117815213&loc=d3e19207-110258 + Details
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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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- Details
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- Details
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- Details
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- Details
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