<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>538</ContextCount>
  <ElementCount>353</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>67</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ConsolidatedStatementsOfIncomeAndComprehensiveIncomeLoss</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME AND COMPREHENSIVE INCOME (LOSS)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - CONSOLIDATED STATEMENTS OF SHAREHOLDERS??? EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ConsolidatedStatementsOfShareholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF SHAREHOLDERS??? EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1003501 - Statement - CONSOLIDATED STATEMENTS OF SHAREHOLDERS??? EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ConsolidatedStatementsOfShareholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF SHAREHOLDERS??? EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SignificantAccountingPolicies</Role>
      <ShortName>Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Allowance for Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLosses</Role>
      <ShortName>Allowance for Loan Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/Securities</Role>
      <ShortName>Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValue</Role>
      <ShortName>Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Short-Term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ShortTermBorrowings</Role>
      <ShortName>Short-Term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - Components of Net Periodic Benefit Cost</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCost</Role>
      <ShortName>Components of Net Periodic Benefit Cost</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - New accounting standards</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/NewAccountingStandards</Role>
      <ShortName>New accounting standards</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Acquisitions and FDIC Indemnification Asset</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAsset</Role>
      <ShortName>Acquisitions and FDIC Indemnification Asset</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Accumulated Other Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncome</Role>
      <ShortName>Accumulated Other Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - Allowance for Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesTables</Role>
      <ShortName>Allowance for Loan Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLosses</ParentRole>
      <Position>17</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/Securities</ParentRole>
      <Position>18</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueTables</Role>
      <ShortName>Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValue</ParentRole>
      <Position>19</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Short-Term Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ShortTermBorrowingsTables</Role>
      <ShortName>Short-Term Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/ShortTermBorrowings</ParentRole>
      <Position>20</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - Components of Net Periodic Benefit Cost (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCostTables</Role>
      <ShortName>Components of Net Periodic Benefit Cost (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCost</ParentRole>
      <Position>21</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Acquisitions and FDIC Indemnification Asset (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAssetTables</Role>
      <ShortName>Acquisitions and FDIC Indemnification Asset (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAsset</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - Accumulated Other Comprehensive Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncome</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2401401 - Disclosure - Significant Accounting Policies (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SignificantAccountingPoliciesDetailsTextual</Role>
      <ShortName>Significant Accounting Policies (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - Allowance for Loan Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetails</Role>
      <ShortName>Allowance for Loan Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - Allowance for Loan Losses (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetails1</Role>
      <ShortName>Allowance for Loan Losses (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Allowance for Loan Losses (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetails2</Role>
      <ShortName>Allowance for Loan Losses (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2402405 - Disclosure - Allowance for Loan Losses (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetails3</Role>
      <ShortName>Allowance for Loan Losses (Details 3)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2402406 - Disclosure - Allowance for Loan Losses Allowance for Loan Losses (Details 4)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesAllowanceForLoanLossesDetails4</Role>
      <ShortName>Allowance for Loan Losses Allowance for Loan Losses (Details 4)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2402407 - Disclosure - Allowance for Loan Losses (Details 5)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetails5</Role>
      <ShortName>Allowance for Loan Losses (Details 5)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2402408 - Disclosure - Allowance for Loan Losses (Details 6)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetails6</Role>
      <ShortName>Allowance for Loan Losses (Details 6)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2402409 - Disclosure - Allowance for Loan Losses (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AllowanceForLoanLossesDetailsTextual</Role>
      <ShortName>Allowance for Loan Losses (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AllowanceForLoanLossesTables</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SecuritiesDetails</Role>
      <ShortName>Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/SecuritiesTables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2403403 - Disclosure - Securities (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SecuritiesDetails1</Role>
      <ShortName>Securities (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/SecuritiesTables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2403404 - Disclosure - Securities (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SecuritiesDetails2</Role>
      <ShortName>Securities (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/SecuritiesTables</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2403405 - Disclosure - Securities (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SecuritiesDetails3</Role>
      <ShortName>Securities (Details 3)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/SecuritiesTables</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2403406 - Disclosure - Securities (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/SecuritiesDetailsTextual</Role>
      <ShortName>Securities (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/SecuritiesTables</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueDetails</Role>
      <ShortName>Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValueTables</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - Fair Value (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueDetails1</Role>
      <ShortName>Fair Value (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValueTables</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - Fair Value (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueDetails2</Role>
      <ShortName>Fair Value (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValueTables</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2404405 - Disclosure - Fair Value (Details 3)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueDetails3</Role>
      <ShortName>Fair Value (Details 3)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValueTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2404406 - Disclosure - Fair Value (Details 4)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueDetails4</Role>
      <ShortName>Fair Value (Details 4)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValueTables</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2404407 - Disclosure - Fair Value (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/FairValueDetailsTextual</Role>
      <ShortName>Fair Value (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/FairValueTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Short-Term Borrowings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ShortTermBorrowingsDetails</Role>
      <ShortName>Short-Term Borrowings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/ShortTermBorrowingsTables</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - Short-Term Borrowings Short-Term Borrowings (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ShortTermBorrowingsShortTermBorrowingsDetails1</Role>
      <ShortName>Short-Term Borrowings Short-Term Borrowings (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - Components of Net Periodic Benefit Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCostDetails</Role>
      <ShortName>Components of Net Periodic Benefit Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCostTables</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Components of Net Periodic Benefit Cost (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCostDetailsTextual</Role>
      <ShortName>Components of Net Periodic Benefit Cost (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/ComponentsOfNetPeriodicBenefitCostTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Acquisitions and FDIC Indemnification Asset (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAssetDetails</Role>
      <ShortName>Acquisitions and FDIC Indemnification Asset (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAssetTables</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2409404 - Disclosure - Acquisitions and FDIC Indemnification Asset (Details Textual)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAssetDetailsTextual</Role>
      <ShortName>Acquisitions and FDIC Indemnification Asset (Details Textual)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AcquisitionsAndFdicIndemnificationAssetTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - Accumulated Other Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2410403 - Disclosure - Accumulated Other Comprehensive Income (Details 1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeDetails1</Role>
      <ShortName>Accumulated Other Comprehensive Income (Details 1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="thff-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Accumulated Other Comprehensive Income (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeDetails2</Role>
      <ShortName>Accumulated Other Comprehensive Income (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.first-online.com/role/AccumulatedOtherComprehensiveIncomeTables</ParentRole>
      <Position>52</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-20200331.xml</File>
    <File>thff-20200331.xsd</File>
    <File>thff-20200331_cal.xml</File>
    <File>thff-20200331_def.xml</File>
    <File>thff-20200331_lab.xml</File>
    <File>thff-20200331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
