<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>522</ContextCount>
  <ElementCount>350</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>85</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>995200090 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DocumentCover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>995200100 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>995200105 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>995200200 - Statement - CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/StatementConsolidatedStatementsOfIncomeAndComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>995200300 - Statement - CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/StatementConsolidatedStatementsOfShareholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>995200305 - Statement - CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/StatementConsolidatedStatementsOfShareholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>995200400 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>995210101 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995210201 - Disclosure - New accounting standards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureNewAccountingStandards</Role>
      <ShortName>New accounting standards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995210301 - Disclosure - Allowance for Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLosses</Role>
      <ShortName>Allowance for Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995210401 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSecurities</Role>
      <ShortName>Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995210501 - Disclosure - Qualified Affordable Housing Project Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureQualifiedAffordableHousingProjectInvestments</Role>
      <ShortName>Qualified Affordable Housing Project Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995210601 - Disclosure - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValue</Role>
      <ShortName>Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995210701 - Disclosure - Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureBorrowings</Role>
      <ShortName>Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995210801 - Disclosure - Components of Net Periodic Benefit Cost</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureComponentsOfNetPeriodicBenefitCost</Role>
      <ShortName>Components of Net Periodic Benefit Cost</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995210901 - Disclosure - Revenue from Contracts with Customers</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureRevenueFromContractsWithCustomers</Role>
      <ShortName>Revenue from Contracts with Customers</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995211001 - Disclosure - Accumulated Other Comprehensive (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLoss</Role>
      <ShortName>Accumulated Other Comprehensive (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995211101 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995211201 - Disclosure - Acquisitions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitions</Role>
      <ShortName>Acquisitions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>99930303 - Disclosure - Allowance for Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesTables</Role>
      <ShortName>Allowance for Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureAllowanceForCreditLosses</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>99930403 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureSecurities</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>99930603 - Disclosure - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValueTables</Role>
      <ShortName>Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureFairValue</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>99930703 - Disclosure - Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureBorrowingsTables</Role>
      <ShortName>Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureBorrowings</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>99930803 - Disclosure - Components of Net Periodic Benefit Cost (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureComponentsOfNetPeriodicBenefitCostTables</Role>
      <ShortName>Components of Net Periodic Benefit Cost (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureComponentsOfNetPeriodicBenefitCost</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>99930903 - Disclosure - Revenue from Contracts with Customers (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureRevenueFromContractsWithCustomersTables</Role>
      <ShortName>Revenue from Contracts with Customers (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureRevenueFromContractsWithCustomers</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>99931003 - Disclosure - Accumulated Other Comprehensive (Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossTables</Role>
      <ShortName>Accumulated Other Comprehensive (Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLoss</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>99931103 - Disclosure - Leases (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureLeasesTables</Role>
      <ShortName>Leases (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureLeases</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>99931203 - Disclosure - Acquisitions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsTables</Role>
      <ShortName>Acquisitions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureAcquisitions</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>99940101 - Disclosure - Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSignificantAccountingPoliciesDetails</Role>
      <ShortName>Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>99940301 - Disclosure - Allowance for Credit Losses - Activity in allowance for credit losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesActivityInAllowanceForCreditLossesDetails</Role>
      <ShortName>Allowance for Credit Losses - Activity in allowance for credit losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>99940302 - Disclosure - Allowance for Credit Losses - Recorded Investments in Nonperforming Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesRecordedInvestmentsInNonperformingLoansDetails</Role>
      <ShortName>Allowance for Credit Losses - Recorded Investments in Nonperforming Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>99940303 - Disclosure - Allowance for Credit Losses- Amortized cost basis of collateral dependent loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesAmortizedCostBasisOfCollateralDependentLoansDetails</Role>
      <ShortName>Allowance for Credit Losses- Amortized cost basis of collateral dependent loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>99940304 - Disclosure - Allowance for Credit Losses - Aging of the Recorded Investment in Loans by Past Due Category and Class of Loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesAgingOfRecordedInvestmentInLoansByPastDueCategoryAndClassOfLoanDetails</Role>
      <ShortName>Allowance for Credit Losses - Aging of the Recorded Investment in Loans by Past Due Category and Class of Loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>99940305 - Disclosure - Allowance for Credit Losses - Loans Modified (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesLoansModifiedDetails</Role>
      <ShortName>Allowance for Credit Losses - Loans Modified (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>99940306 - Disclosure - Allowance for Credit Losses - Modified loans performance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesModifiedLoansPerformanceDetails</Role>
      <ShortName>Allowance for Credit Losses - Modified loans performance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>99940307 - Disclosure - Allowance for Credit Losses - Effect of loan and leases modifications (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesEffectOfLoanAndLeasesModificationsDetails</Role>
      <ShortName>Allowance for Credit Losses - Effect of loan and leases modifications (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>99940308 - Disclosure - Allowance for Credit Losses - Modified loans with payment default (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesModifiedLoansWithPaymentDefaultDetails</Role>
      <ShortName>Allowance for Credit Losses - Modified loans with payment default (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>99940309 - Disclosure - Allowance for Credit Losses - Recorded Investment of Commercial Loan Portfolio by Risk Category (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesRecordedInvestmentOfCommercialLoanPortfolioByRiskCategoryDetails</Role>
      <ShortName>Allowance for Credit Losses - Recorded Investment of Commercial Loan Portfolio by Risk Category (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>99940310 - Disclosure - Allowance for Credit Losses - Credit risk profile of loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesCreditRiskProfileOfLoansDetails</Role>
      <ShortName>Allowance for Credit Losses - Credit risk profile of loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>99940311 - Disclosure - Allowance for Credit Losses - Purchased Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAllowanceForCreditLossesPurchasedLoansDetails</Role>
      <ShortName>Allowance for Credit Losses - Purchased Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>99940401 - Disclosure - Securities - Amortized cost and fair value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSecuritiesAmortizedCostAndFairValueDetails</Role>
      <ShortName>Securities - Amortized cost and fair value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>99940402 - Disclosure - Securities - Contractual maturities of debt securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSecuritiesContractualMaturitiesOfDebtSecuritiesDetails</Role>
      <ShortName>Securities - Contractual maturities of debt securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>99940403 - Disclosure - Securities - Securities Gross (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSecuritiesSecuritiesGrossDetails</Role>
      <ShortName>Securities - Securities Gross (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>99940404 - Disclosure - Securities - Roll forward of the credit losses recognized in earnings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureSecuritiesRollForwardOfCreditLossesRecognizedInEarningsDetails</Role>
      <ShortName>Securities - Roll forward of the credit losses recognized in earnings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>99940501 - Disclosure - Qualified Affordable Housing Project Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureQualifiedAffordableHousingProjectInvestmentsDetails</Role>
      <ShortName>Qualified Affordable Housing Project Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureQualifiedAffordableHousingProjectInvestments</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>99940601 - Disclosure - Fair Value - Fair value of derivatives is based on valuation models using observable market data as of the measurement date (Level 2 inputs) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValueFairValueOfDerivativesIsBasedOnValuationModelsUsingObservableMarketDataAsOfMeasurementDateLevel2InputsDetails</Role>
      <ShortName>Fair Value - Fair value of derivatives is based on valuation models using observable market data as of the measurement date (Level 2 inputs) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>99940602 - Disclosure - Fair Value - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValueAdditionalInformationDetails</Role>
      <ShortName>Fair Value - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>99940603 - Disclosure - Fair Value - Reconciliation and income statement classification of gains and losses for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValueReconciliationAndIncomeStatementClassificationOfGainsAndLossesForAllAssetsMeasuredAtFairValueOnRecurringBasisUsingSignificantUnobservableInputsLevel3Details</Role>
      <ShortName>Fair Value - Reconciliation and income statement classification of gains and losses for all assets measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>99940604 - Disclosure - Fair Value - Quantitative information about recurring and non-recurring Level 3 fair value measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValueQuantitativeInformationAboutRecurringAndNonRecurringLevel3FairValueMeasurementsDetails</Role>
      <ShortName>Fair Value - Quantitative information about recurring and non-recurring Level 3 fair value measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>99940605 - Disclosure - Fair Value - Carrying amounts and estimated fair value of financial instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureFairValueCarryingAmountsAndEstimatedFairValueOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value - Carrying amounts and estimated fair value of financial instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>99940701 - Disclosure - Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureBorrowingsDetails</Role>
      <ShortName>Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureBorrowingsTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>99940702 - Disclosure - Borrowings - Repurchase Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureBorrowingsRepurchaseAgreementsDetails</Role>
      <ShortName>Borrowings - Repurchase Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>99940703 - Disclosure - Borrowings - Schedule of Other Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureBorrowingsScheduleOfOtherBorrowingsDetails</Role>
      <ShortName>Borrowings - Schedule of Other Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>99940704 - Disclosure - Borrowings - Aggregate Minimum Annual Retirements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureBorrowingsAggregateMinimumAnnualRetirementsDetails</Role>
      <ShortName>Borrowings - Aggregate Minimum Annual Retirements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>99940801 - Disclosure - Components of Net Periodic Benefit Cost - Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureComponentsOfNetPeriodicBenefitCostNetPeriodicBenefitCostDetails</Role>
      <ShortName>Components of Net Periodic Benefit Cost - Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>99940802 - Disclosure - Components of Net Periodic Benefit Cost - Employer Contributions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureComponentsOfNetPeriodicBenefitCostEmployerContributionsDetails</Role>
      <ShortName>Components of Net Periodic Benefit Cost - Employer Contributions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>99940901 - Disclosure - Revenue from Contracts with Customers (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureRevenueFromContractsWithCustomersDetails</Role>
      <ShortName>Revenue from Contracts with Customers (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureRevenueFromContractsWithCustomersTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>99941001 - Disclosure - Accumulated Other Comprehensive (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>99941002 - Disclosure - Accumulated Other Comprehensive (Loss) - Unrealized gains (losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossUnrealizedGainsLossesDetails</Role>
      <ShortName>Accumulated Other Comprehensive (Loss) - Unrealized gains (losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>99941003 - Disclosure - Accumulated Other Comprehensive (Loss) - Amount reclassified from accumulated other comprehensive income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossAmountReclassifiedFromAccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Accumulated Other Comprehensive (Loss) - Amount reclassified from accumulated other comprehensive income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureAccumulatedOtherComprehensiveLossTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>99941101 - Disclosure - Leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureLeasesDetails</Role>
      <ShortName>Leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/DisclosureLeasesTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>99941102 - Disclosure - Leases - Lease cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureLeasesLeaseCostDetails</Role>
      <ShortName>Leases - Lease cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>99941103 - Disclosure - Leases - Future minimum payments for operating leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureLeasesFutureMinimumPaymentsForOperatingLeasesDetails</Role>
      <ShortName>Leases - Future minimum payments for operating leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>99941201 - Disclosure - Acquisitions - CedarStone Assets acquired and liabilities assumed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsCedarstoneAssetsAcquiredAndLiabilitiesAssumedDetails</Role>
      <ShortName>Acquisitions - CedarStone Assets acquired and liabilities assumed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>99941202 - Disclosure - Acquisitions - CedarStone Financial Assets With Credit Deterioration (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsCedarstoneFinancialAssetsWithCreditDeteriorationDetails</Role>
      <ShortName>Acquisitions - CedarStone Financial Assets With Credit Deterioration (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>99941203 - Disclosure - Acquisitions - CedarStone Pro forma Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsCedarstoneProFormaFinancialInformationDetails</Role>
      <ShortName>Acquisitions - CedarStone Pro forma Financial Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>99941204 - Disclosure - Acquisitions - SimplyBank Assets acquired and liabilities assumed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsSimplybankAssetsAcquiredAndLiabilitiesAssumedDetails</Role>
      <ShortName>Acquisitions - SimplyBank Assets acquired and liabilities assumed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>99941205 - Disclosure - Acquisitions - SimplyBank Financial Assets With Credit Deterioration (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsSimplybankFinancialAssetsWithCreditDeteriorationDetails</Role>
      <ShortName>Acquisitions - SimplyBank Financial Assets With Credit Deterioration (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="thff-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>99941206 - Disclosure - Acquisitions - SimplyBank Pro forma Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DisclosureAcquisitionsSimplybankProFormaFinancialInformationDetails</Role>
      <ShortName>Acquisitions - SimplyBank Pro forma Financial Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>thff-20260331.xsd</File>
    <File>thff-20260331_cal.xml</File>
    <File>thff-20260331_def.xml</File>
    <File>thff-20260331_lab.xml</File>
    <File>thff-20260331_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="thff-20260331x10q.htm">thff-20260331x10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1752">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
