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Components of Net Periodic Benefit Cost (Details Textual) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  | $ 1,800,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths
Defined Benefit Plan, Expected Future Benefit Payments, Next Rolling Twelve Months |  |  | 1,100,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextRollingTwelveMonths
Defined Contribution Plan Employer Accrued Discretionary Contribution Amount | 359,000 thff_DefinedContributionPlanEmployerAccruedDiscretionaryContributionAmount | 350,000 thff_DefinedContributionPlanEmployerAccruedDiscretionaryContributionAmount |
Pension Benefits [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contributions made by employer | 0 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_RegulatoryLiabilityAxis = thff_PensionBenefitsMember |  |
Post-Retirement Health Benefits [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  | 247,000 us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextTwelveMonths / us-gaap_RegulatoryLiabilityAxis = thff_PostRetirementHealthBenefitsMember
Contributions made by employer | $ 57,000 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_RegulatoryLiabilityAxis = thff_PostRetirementHealthBenefitsMember |  |

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- Definition Amount of potential discretionary contributions accrued to a defined contribution plan. + References No definition available. + Details
Name: | thff_DefinedContributionPlanEmployerAccruedDiscretionaryContributionAmount
Namespace Prefix: | thff_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of benefits from a defined benefit plan expected to be paid in the next rolling twelve months following the latest balance sheet. For interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanExpectedFutureBenefitPaymentsNextRollingTwelveMonths
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of benefits from a defined benefit plan expected to be paid in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_RegulatoryLiabilityAxis=thff_PensionBenefitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RegulatoryLiabilityAxis=thff_PostRetirementHealthBenefitsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |