<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.10.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>468</ContextCount>
  <ElementCount>690</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>103</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information Document</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DocumentAndEntityInformationDocument</Role>
      <ShortName>Document and Entity Information Document</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001001 - Statement - CONSOLIDATED BALANCE SHEETS CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedBalanceSheetsConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004000 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedStatementsOfChangesInShareholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004001 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedStatementsOfChangesInShareholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1005000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.bankatfirst.com/role/SummaryOfSignificantAccountingPoliciesNotes</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - RECENTLY ADOPTED AND ISSUED ACCOUNTING STANDARDS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RecentlyAdoptedAndIssuedAccountingStandards</Role>
      <ShortName>RECENTLY ADOPTED AND ISSUED ACCOUNTING STANDARDS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - RESTRICTIONS ON CASH AND DIVIDENDS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RestrictionsOnCashAndDividends</Role>
      <ShortName>RESTRICTIONS ON CASH AND DIVIDENDS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - INVESTMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/Investments</Role>
      <ShortName>INVESTMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - LOANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/Loans</Role>
      <ShortName>LOANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - ALLOWANCE FOR LOAN AND LEASE LOSSES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AllowanceForLoanAndLeaseLosses</Role>
      <ShortName>ALLOWANCE FOR LOAN AND LEASE LOSSES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - PREMISES AND EQUIPMENT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/PremisesAndEquipment</Role>
      <ShortName>PREMISES AND EQUIPMENT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/GoodwillAndOtherIntangibleAssets</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - DEPOSITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/Deposits</Role>
      <ShortName>DEPOSITS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/Borrowings</Role>
      <ShortName>BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - DERIVATIVES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/Derivatives</Role>
      <ShortName>DERIVATIVES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - RELATED PARTIES TRANSACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RelatedPartiesTransactions</Role>
      <ShortName>RELATED PARTIES TRANSACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/CommitmentsAndContingencies</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/IncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - EMPLOYEE BENEFIT PLANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EmployeeBenefitPlans</Role>
      <ShortName>EMPLOYEE BENEFIT PLANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - REVENUE RECOGNITION REVENUE RECOGNITION (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.bankatfirst.com/role/RevenueRecognitionRevenueRecognitionNotes</Role>
      <ShortName>REVENUE RECOGNITION REVENUE RECOGNITION (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2118100 - Disclosure - CAPITAL</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/Capital</Role>
      <ShortName>CAPITAL</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2119100 - Disclosure - STOCK OPTIONS AND AWARDS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/StockOptionsAndAwards</Role>
      <ShortName>STOCK OPTIONS AND AWARDS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2120100 - Disclosure - EARNINGS PER COMMON SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EarningsPerCommonShare</Role>
      <ShortName>EARNINGS PER COMMON SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - FAIR VALUE DISCLOSURES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FairValueDisclosures</Role>
      <ShortName>FAIR VALUE DISCLOSURES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2122100 - Disclosure - BUSINESS COMBINATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BusinessCombinations</Role>
      <ShortName>BUSINESS COMBINATIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2123100 - Disclosure - FIRST FINANCIAL BANCORP. (PARENT COMPANY ONLY) FINANCIAL INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FirstFinancialBancorpParentCompanyOnlyFinancialInformation</Role>
      <ShortName>FIRST FINANCIAL BANCORP. (PARENT COMPANY ONLY) FINANCIAL INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - INVESTMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/InvestmentsTables</Role>
      <ShortName>INVESTMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/Investments</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - LOANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansTables</Role>
      <ShortName>LOANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/Loans</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2306301 - Disclosure - ALLOWANCE FOR LOAN AND LEASE LOSSES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AllowanceForLoanAndLeaseLossesTables</Role>
      <ShortName>ALLOWANCE FOR LOAN AND LEASE LOSSES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/AllowanceForLoanAndLeaseLosses</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - PREMISES AND EQUIPMENT (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/PremisesAndEquipmentTables</Role>
      <ShortName>PREMISES AND EQUIPMENT (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/PremisesAndEquipment</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/GoodwillAndOtherIntangibleAssetsGoodwillTables</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/GoodwillAndOtherIntangibleAssets</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - DEPOSITS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DepositsTables</Role>
      <ShortName>DEPOSITS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/Deposits</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BorrowingsTables</Role>
      <ShortName>BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/Borrowings</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - DERIVATIVES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DerivativesTables</Role>
      <ShortName>DERIVATIVES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/Derivatives</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - RELATED PARTIES TRANSACTIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RelatedPartiesTransactionsTables</Role>
      <ShortName>RELATED PARTIES TRANSACTIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/RelatedPartiesTransactions</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2314301 - Disclosure - INCOME TAXES INCOME TAXES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/IncomeTaxesIncomeTaxesTables</Role>
      <ShortName>INCOME TAXES INCOME TAXES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - EMPLOYEE BENEFIT PLANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EmployeeBenefitPlansTables</Role>
      <ShortName>EMPLOYEE BENEFIT PLANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/EmployeeBenefitPlans</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2317301 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/AccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2318301 - Disclosure - CAPITAL (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/CapitalTables</Role>
      <ShortName>CAPITAL (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/Capital</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2319301 - Disclosure - STOCK OPTIONS AND AWARDS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/StockOptionsAndAwardsTables</Role>
      <ShortName>STOCK OPTIONS AND AWARDS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/StockOptionsAndAwards</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2320301 - Disclosure - EARNINGS PER COMMON SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EarningsPerCommonShareTables</Role>
      <ShortName>EARNINGS PER COMMON SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/EarningsPerCommonShare</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2321301 - Disclosure - FAIR VALUE DISCLOSURES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FairValueDisclosuresTables</Role>
      <ShortName>FAIR VALUE DISCLOSURES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/FairValueDisclosures</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2322301 - Disclosure - BUSINESS COMBINATIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BusinessCombinationsTables</Role>
      <ShortName>BUSINESS COMBINATIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/BusinessCombinations</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2323301 - Disclosure - FIRST FINANCIAL BANCORP. (PARENT COMPANY ONLY) FINANCIAL INFORMATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FirstFinancialBancorpParentCompanyOnlyFinancialInformationTables</Role>
      <ShortName>FIRST FINANCIAL BANCORP. (PARENT COMPANY ONLY) FINANCIAL INFORMATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/FirstFinancialBancorpParentCompanyOnlyFinancialInformation</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2401402 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/SummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/SummaryOfSignificantAccountingPoliciesSummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Premises and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/SummaryOfSignificantAccountingPoliciesPremisesAndEquipmentDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Premises and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2402401 - Disclosure - RECENTLY ADOPTED AND ISSUED ACCOUNTING STANDARDS RECENTLY ADOPTED AND ISSUED ACCOUNTING STANDARDS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RecentlyAdoptedAndIssuedAccountingStandardsRecentlyAdoptedAndIssuedAccountingStandardsAdditionalInformationDetails</Role>
      <ShortName>RECENTLY ADOPTED AND ISSUED ACCOUNTING STANDARDS RECENTLY ADOPTED AND ISSUED ACCOUNTING STANDARDS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2403401 - Disclosure - RESTRICTIONS ON CASH AND DIVIDENDS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RestrictionsOnCashAndDividendsDetails</Role>
      <ShortName>RESTRICTIONS ON CASH AND DIVIDENDS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/RestrictionsOnCashAndDividends</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - INVESTMENT SECURITIES INVESTMENTS - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/InvestmentSecuritiesInvestmentsAdditionalInformationDetails</Role>
      <ShortName>INVESTMENT SECURITIES INVESTMENTS - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/InvestmentsSummaryOfHeldToMaturityAndAvailableForSaleInvestmentSecuritiesDetail</Role>
      <ShortName>INVESTMENTS - Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/InvestmentsSummaryOfInvestmentSecuritiesByEstimatedMaturityDetail</Role>
      <ShortName>INVESTMENTS - Summary of Investment Securities by Estimated Maturity (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2404405 - Disclosure - INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/InvestmentsAgeOfGrossUnrealizedLossesAndAssociatedFairValueByInvestmentCategoryDetail</Role>
      <ShortName>INVESTMENTS - Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - LOANS - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansAdditionalInformationDetail</Role>
      <ShortName>LOANS - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - LOANS - Commercial and Consumer Credit Exposure by Risk Attribute (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansCommercialAndConsumerCreditExposureByRiskAttributeDetail</Role>
      <ShortName>LOANS - Commercial and Consumer Credit Exposure by Risk Attribute (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - LOANS - Loan Delinquency, including Nonaccrual Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansLoanDelinquencyIncludingNonaccrualLoansDetail</Role>
      <ShortName>LOANS - Loan Delinquency, including Nonaccrual Loans (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2405405 - Disclosure - LOANS - Restructured Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansRestructuredLoansDetails</Role>
      <ShortName>LOANS - Restructured Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - LOANS - Nonaccrual, Restructured and Impaired Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansNonaccrualRestructuredAndImpairedLoansDetail</Role>
      <ShortName>LOANS - Nonaccrual, Restructured and Impaired Loans (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2405407 - Disclosure - LOANS - Investment in Impaired Loans (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansInvestmentInImpairedLoansDetail</Role>
      <ShortName>LOANS - Investment in Impaired Loans (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2405408 - Disclosure - LOANS - Changes in Other Real Estate Owned (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/LoansChangesInOtherRealEstateOwnedDetail</Role>
      <ShortName>LOANS - Changes in Other Real Estate Owned (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - ALLOWANCE FOR LOAN AND LEASE LOSSES - Changes in the Allowance for Loan and Lease Losses for the Previous Three Years (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AllowanceForLoanAndLeaseLossesChangesInAllowanceForLoanAndLeaseLossesForPreviousThreeYearsDetail</Role>
      <ShortName>ALLOWANCE FOR LOAN AND LEASE LOSSES - Changes in the Allowance for Loan and Lease Losses for the Previous Three Years (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - ALLOWANCE FOR LOAN AND LEASE LOSSES - Changes in the Allowance for Loan and Lease Losses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AllowanceForLoanAndLeaseLossesChangesInAllowanceForLoanAndLeaseLossesDetail</Role>
      <ShortName>ALLOWANCE FOR LOAN AND LEASE LOSSES - Changes in the Allowance for Loan and Lease Losses (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - PREMISES AND EQUIPMENT - Schedule of Premises and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/PremisesAndEquipmentScheduleOfPremisesAndEquipmentDetails</Role>
      <ShortName>PREMISES AND EQUIPMENT - Schedule of Premises and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - PREMISES AND EQUIPMENT - Operating leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/PremisesAndEquipmentOperatingLeasesDetails</Role>
      <ShortName>PREMISES AND EQUIPMENT - Operating leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL AND OTHER INTANGIBLE ASSETS-Schedule of Goodwill (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/GoodwillAndOtherIntangibleAssetsGoodwillAndOtherIntangibleAssetsScheduleOfGoodwillDetails</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL AND OTHER INTANGIBLE ASSETS-Schedule of Goodwill (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL AND OTHER INTANGIBLE ASSETS-Schedule of Future Amortization Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/GoodwillAndOtherIntangibleAssetsGoodwillAndOtherIntangibleAssetsScheduleOfFutureAmortizationExpenseDetails</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL AND OTHER INTANGIBLE ASSETS-Schedule of Future Amortization Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/GoodwillAndOtherIntangibleAssetsGoodwillAndOtherIntangibleAssetsAdditionalInformationDetails</Role>
      <ShortName>GOODWILL AND OTHER INTANGIBLE ASSETS GOODWILL AND OTHER INTANGIBLE ASSETS--Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - DEPOSITS DEPOSITS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DepositsDepositsDetails</Role>
      <ShortName>DEPOSITS DEPOSITS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - DEPOSITS DEPOSITS-Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DepositsDepositsAdditionalInformationDetails</Role>
      <ShortName>DEPOSITS DEPOSITS-Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - BORROWINGS BORROWINGS - Repurchase Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BorrowingsBorrowingsRepurchaseAgreementsDetails</Role>
      <ShortName>BORROWINGS BORROWINGS - Repurchase Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2410403 - Disclosure - BORROWINGS - Schedule of Short-term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BorrowingsScheduleOfShortTermDebtDetails</Role>
      <ShortName>BORROWINGS - Schedule of Short-term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - BORROWINGS - Schedule of Long-term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BorrowingsScheduleOfLongTermDebtDetails</Role>
      <ShortName>BORROWINGS - Schedule of Long-term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2410405 - Disclosure - BORROWINGS Borrowings - - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BorrowingsBorrowingsAdditionalInformationDetails</Role>
      <ShortName>BORROWINGS Borrowings - - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - DERIVATIVES - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DerivativesAdditionalInformationDetail</Role>
      <ShortName>DERIVATIVES - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2411403 - Disclosure - DERIVATIVES - Summary of Derivative Financial Instruments and Balances (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DerivativesSummaryOfDerivativeFinancialInstrumentsAndBalancesDetail</Role>
      <ShortName>DERIVATIVES - Summary of Derivative Financial Instruments and Balances (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2411404 - Disclosure - DERIVATIVES DERIVATIVES - Disclosure by Type of Financial Instrument (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DerivativesDerivativesDisclosureByTypeOfFinancialInstrumentDetails</Role>
      <ShortName>DERIVATIVES DERIVATIVES - Disclosure by Type of Financial Instrument (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2411405 - Disclosure - DERIVATIVES - Derivative Financial Instruments, Average Remaining Maturity and the Weighted-Average Interest Rates being Paid and Received (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/DerivativesDerivativeFinancialInstrumentsAverageRemainingMaturityAndWeightedAverageInterestRatesBeingPaidAndReceivedDetail</Role>
      <ShortName>DERIVATIVES - Derivative Financial Instruments, Average Remaining Maturity and the Weighted-Average Interest Rates being Paid and Received (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - RELATED PARTIES TRANSACTIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RelatedPartiesTransactionsDetails</Role>
      <ShortName>RELATED PARTIES TRANSACTIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/RelatedPartiesTransactionsTables</ParentRole>
      <Position>83</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2413401 - Disclosure - COMMITMENTS AND CONTINGENCIES (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/CommitmentsAndContingenciesDetail</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2414402 - Disclosure - INCOME TAXES (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/IncomeTaxesDetail</Role>
      <ShortName>INCOME TAXES (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/IncomeTaxesIncomeTaxesTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EmployeeBenefitPlansEmployeeBenefitPlansDetails</Role>
      <ShortName>EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - EMPLOYEE BENEFIT PLANS Defined Benefit Plan, Fair Value on Plan Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EmployeeBenefitPlansDefinedBenefitPlanFairValueOnPlanAssetsDetails</Role>
      <ShortName>EMPLOYEE BENEFIT PLANS Defined Benefit Plan, Fair Value on Plan Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2416402 - Disclosure - REVENUE RECOGNITION REVENUE RECOGNITION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/RevenueRecognitionRevenueRecognitionDetails</Role>
      <ShortName>REVENUE RECOGNITION REVENUE RECOGNITION (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/RevenueRecognitionRevenueRecognitionNotes</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2417402 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/AccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2417403 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME AMOUNT RECLASSIFIED FROM ACCUMULATED OTHER COMPREHENSIVE INCOME (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/AccumulatedOtherComprehensiveIncomeAmountReclassifiedFromAccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME AMOUNT RECLASSIFIED FROM ACCUMULATED OTHER COMPREHENSIVE INCOME (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2418402 - Disclosure - CAPITAL - Risk-Based Capital (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/CapitalRiskBasedCapitalDetails</Role>
      <ShortName>CAPITAL - Risk-Based Capital (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2418403 - Disclosure - CAPITAL - Share Repurchase (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/CapitalShareRepurchaseDetails</Role>
      <ShortName>CAPITAL - Share Repurchase (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2419402 - Disclosure - STOCK OPTIONS AND AWARDS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/StockOptionsAndAwardsDetails</Role>
      <ShortName>STOCK OPTIONS AND AWARDS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/StockOptionsAndAwardsTables</ParentRole>
      <Position>93</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2420402 - Disclosure - EARNINGS PER COMMON SHARE - Computation of Basic and Diluted Earnings Per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EarningsPerCommonShareComputationOfBasicAndDilutedEarningsPerShareDetail</Role>
      <ShortName>EARNINGS PER COMMON SHARE - Computation of Basic and Diluted Earnings Per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2420403 - Disclosure - EARNINGS PER COMMON SHARE - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/EarningsPerCommonShareAdditionalInformationDetail</Role>
      <ShortName>EARNINGS PER COMMON SHARE - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2421402 - Disclosure - FAIR VALUE DISCLOSURES - Estimated Fair Values of Financial Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FairValueDisclosuresEstimatedFairValuesOfFinancialInstrumentsDetail</Role>
      <ShortName>FAIR VALUE DISCLOSURES - Estimated Fair Values of Financial Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2421403 - Disclosure - FAIR VALUE DISCLOSURES - Summary of Financial Assets and Liabilities Measure at Fair Value on a Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FairValueDisclosuresSummaryOfFinancialAssetsAndLiabilitiesMeasureAtFairValueOnRecurringBasisDetail</Role>
      <ShortName>FAIR VALUE DISCLOSURES - Summary of Financial Assets and Liabilities Measure at Fair Value on a Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2421404 - Disclosure - FAIR VALUE DISCLOSURES - Summary of Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FairValueDisclosuresSummaryOfFinancialAssetsAndLiabilitiesMeasuredAtFairValueOnNonrecurringBasisDetail</Role>
      <ShortName>FAIR VALUE DISCLOSURES - Summary of Financial Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2422402 - Disclosure - BUSINESS COMBINATIONS - Additional Information - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BusinessCombinationsAdditionalInformationNarrativeDetails</Role>
      <ShortName>BUSINESS COMBINATIONS - Additional Information - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2422403 - Disclosure - BUSINESS COMBINATION BUSINESS COMBINATIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BusinessCombinationBusinessCombinationsDetails</Role>
      <ShortName>BUSINESS COMBINATION BUSINESS COMBINATIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2422404 - Disclosure - BUSINESS COMBINATION BUSINESS COMBINATIONS - Pro Forma Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/BusinessCombinationBusinessCombinationsProFormaInformationDetails</Role>
      <ShortName>BUSINESS COMBINATION BUSINESS COMBINATIONS - Pro Forma Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="ffbc-20181231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2423402 - Disclosure - FIRST FINANCIAL BANCORP. (PARENT COMPANY ONLY) FINANCIAL INFORMATION (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankatfirst.com/role/FirstFinancialBancorpParentCompanyOnlyFinancialInformationDetails</Role>
      <ShortName>FIRST FINANCIAL BANCORP. (PARENT COMPANY ONLY) FINANCIAL INFORMATION (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankatfirst.com/role/FirstFinancialBancorpParentCompanyOnlyFinancialInformationTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>ffbc-20181231.xml</File>
    <File>ffbc-20181231.xsd</File>
    <File>ffbc-20181231_cal.xml</File>
    <File>ffbc-20181231_def.xml</File>
    <File>ffbc-20181231_lab.xml</File>
    <File>ffbc-20181231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2018-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2018-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
