<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.8.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1019</ContextCount>
  <ElementCount>734</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>184</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>13</UnitCount>
  <MyReports>
    <Report instance="fcf-20171231.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.fcbanking.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ConsolidatedStatementsOfFinancialCondition</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ConsolidatedStatementsOfFinancialConditionParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fcf-20171231.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.fcbanking.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fcf-20171231.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.fcbanking.com/role/ConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fcf-20171231.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.fcbanking.com/role/ConsolidatedStatementsOfChangesInShareholdersEquity</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004501 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ConsolidatedStatementsOfChangesInShareholdersEquityParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fcf-20171231.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.fcbanking.com/role/ConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Statement of Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/StatementOfAccountingPolicies</Role>
      <ShortName>Statement of Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - New Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/NewAccountingPronouncements</Role>
      <ShortName>New Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Acquisition</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/Acquisition</Role>
      <ShortName>Acquisition</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - Supplemental Comprehensive Income Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosures</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Supplemental Cash Flow Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalCashFlowDisclosures</Role>
      <ShortName>Supplemental Cash Flow Disclosures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2122100 - Disclosure - Earnings per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/EarningsPerShare</Role>
      <ShortName>Earnings per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2125100 - Disclosure - Cash and Due from Banks</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CashAndDueFromBanks</Role>
      <ShortName>Cash and Due from Banks</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2130100 - Disclosure - Derivatives</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/Derivatives</Role>
      <ShortName>Derivatives</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2134100 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecurities</Role>
      <ShortName>Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2135100 - Disclosure - Impairment of Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecurities</Role>
      <ShortName>Impairment of Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2140100 - Disclosure - Loans and Allowance for Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLosses</Role>
      <ShortName>Loans and Allowance for Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2141100 - Disclosure - Commitments and Letters of Credit</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsAndLettersOfCredit</Role>
      <ShortName>Commitments and Letters of Credit</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2144100 - Disclosure - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/PremisesAndEquipment</Role>
      <ShortName>Premises and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2145100 - Disclosure - Goodwill and Other Amortizing Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/GoodwillAndOtherAmortizingIntangibleAssets</Role>
      <ShortName>Goodwill and Other Amortizing Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2146100 - Disclosure - Interest-Bearing Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InterestBearingDeposits</Role>
      <ShortName>Interest-Bearing Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2147100 - Disclosure - Short-term Borrowings</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ShortTermBorrowings</Role>
      <ShortName>Short-term Borrowings</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2148100 - Disclosure - Subordinated Debentures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebentures</Role>
      <ShortName>Subordinated Debentures</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2149100 - Disclosure - Other Long-term Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/OtherLongTermDebt</Role>
      <ShortName>Other Long-term Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2150100 - Disclosure - Fair Values of Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilities</Role>
      <ShortName>Fair Values of Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2151100 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2153100 - Disclosure - Retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlans</Role>
      <ShortName>Retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2154100 - Disclosure - Incentive Compensation Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncentiveCompensationPlan</Role>
      <ShortName>Incentive Compensation Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2155100 - Disclosure - Contingent Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ContingentLiabilities</Role>
      <ShortName>Contingent Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2156100 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2157100 - Disclosure - Regulatory Restrictions and Capital Adequacy</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RegulatoryRestrictionsAndCapitalAdequacy</Role>
      <ShortName>Regulatory Restrictions and Capital Adequacy</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2158100 - Disclosure - Capital</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/Capital</Role>
      <ShortName>Capital</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2159100 - Disclosure - Condensed Financial Information of First Commonwealth Financial Corporation (parent company only)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnly</Role>
      <ShortName>Condensed Financial Information of First Commonwealth Financial Corporation (parent company only)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2160100 - Disclosure - Subsequent Event (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.fcbanking.com/role/SubsequentEventNotes</Role>
      <ShortName>Subsequent Event (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - Statement of Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/StatementOfAccountingPoliciesPolicies</Role>
      <ShortName>Statement of Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/StatementOfAccountingPolicies</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Acquisition (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/AcquisitionTables</Role>
      <ShortName>Acquisition (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/Acquisition</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - Supplemental Comprehensive Income Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosuresTables</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosures</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2313301 - Disclosure - Supplemental Cash Flow Disclosures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalCashFlowDisclosuresTables</Role>
      <ShortName>Supplemental Cash Flow Disclosures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/SupplementalCashFlowDisclosures</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2322301 - Disclosure - Earnings per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/EarningsPerShare</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2330301 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/DerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/Derivatives</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2334301 - Disclosure - Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesTables</Role>
      <ShortName>Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/InvestmentSecurities</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2335301 - Disclosure - Impairment of Investment Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecuritiesTables</Role>
      <ShortName>Impairment of Investment Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecurities</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2340301 - Disclosure - Loans and Allowance for Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesTables</Role>
      <ShortName>Loans and Allowance for Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLosses</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2341301 - Disclosure - Commitments and Letters of Credit (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsAndLettersOfCreditTables</Role>
      <ShortName>Commitments and Letters of Credit (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CommitmentsAndLettersOfCredit</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2344301 - Disclosure - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/PremisesAndEquipmentTables</Role>
      <ShortName>Premises and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/PremisesAndEquipment</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2345301 - Disclosure - Goodwill and Other Amortizing Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/GoodwillAndOtherAmortizingIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Amortizing Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/GoodwillAndOtherAmortizingIntangibleAssets</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2346301 - Disclosure - Interest-Bearing Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InterestBearingDepositsTables</Role>
      <ShortName>Interest-Bearing Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/InterestBearingDeposits</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2347301 - Disclosure - Short-term Borrowings (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ShortTermBorrowingsTables</Role>
      <ShortName>Short-term Borrowings (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/ShortTermBorrowings</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2348301 - Disclosure - Subordinated Debentures (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesTables</Role>
      <ShortName>Subordinated Debentures (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/SubordinatedDebentures</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2349301 - Disclosure - Other Long-term Debt (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/OtherLongTermDebtTables</Role>
      <ShortName>Other Long-term Debt (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/OtherLongTermDebt</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2350301 - Disclosure - Fair Values of Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesTables</Role>
      <ShortName>Fair Values of Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilities</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2351301 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/IncomeTaxes</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2353301 - Disclosure - Retirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansTables</Role>
      <ShortName>Retirement Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/RetirementPlans</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2354301 - Disclosure - Incentive Compensation Plan (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncentiveCompensationPlanTables</Role>
      <ShortName>Incentive Compensation Plan (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/IncentiveCompensationPlan</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2356301 - Disclosure - Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/RelatedPartyTransactions</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2357301 - Disclosure - Regulatory Restrictions and Capital Adequacy (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RegulatoryRestrictionsAndCapitalAdequacyTables</Role>
      <ShortName>Regulatory Restrictions and Capital Adequacy (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/RegulatoryRestrictionsAndCapitalAdequacy</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2359301 - Disclosure - Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyTables</Role>
      <ShortName>Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnly</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2401402 - Disclosure - Statement of Accounting Policies - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/StatementOfAccountingPoliciesAdditionalInformationDetail</Role>
      <ShortName>Statement of Accounting Policies - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Acquisition - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/AcquisitionNarrativeDetails</Role>
      <ShortName>Acquisition - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - Acquisition - Summary (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/AcquisitionSummaryDetails</Role>
      <ShortName>Acquisition - Summary (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosuresRelatedTaxEffectsAllocatedToEachComponentOfOtherComprehensiveIncomeDetail</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - Supplemental Comprehensive Income Disclosures Supplemental Comprehensive Income Disclosures (Details 2)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosuresSupplementalComprehensiveIncomeDisclosuresDetails2</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures Supplemental Comprehensive Income Disclosures (Details 2)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2413402 - Disclosure - Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalCashFlowDisclosuresNonCashInvestingAndFinancingActivitiesDetail</Role>
      <ShortName>Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2422402 - Disclosure - Earnings per Share - Composition of Weighted-Average Common Shares (Denominator) Used in Basic and Diluted Earnings Per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/EarningsPerShareCompositionOfWeightedAverageCommonSharesDenominatorUsedInBasicAndDilutedEarningsPerShareDetail</Role>
      <ShortName>Earnings per Share - Composition of Weighted-Average Common Shares (Denominator) Used in Basic and Diluted Earnings Per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2422403 - Disclosure - Earnings per Share - Common Stock Equivalents Not Included in Computation of Diluted Earnings Per Share (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/EarningsPerShareCommonStockEquivalentsNotIncludedInComputationOfDilutedEarningsPerShareDetail</Role>
      <ShortName>Earnings per Share - Common Stock Equivalents Not Included in Computation of Diluted Earnings Per Share (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2425401 - Disclosure - Cash and Due from Banks - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CashAndDueFromBanksAdditionalInformationDetail</Role>
      <ShortName>Cash and Due from Banks - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2430402 - Disclosure - Derivatives - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/DerivativesAdditionalInformationDetail</Role>
      <ShortName>Derivatives - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2430403 - Disclosure - Derivatives - Credit Value Adjustment Recorded Related to Notional Amount of Derivatives Outstanding (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/DerivativesCreditValueAdjustmentRecordedRelatedToNotionalAmountOfDerivativesOutstandingDetail</Role>
      <ShortName>Derivatives - Credit Value Adjustment Recorded Related to Notional Amount of Derivatives Outstanding (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2430404 - Disclosure - Derivatives - Schedule of Change in Fair Value of Derivative Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/DerivativesScheduleOfChangeInFairValueOfDerivativeAssetsAndLiabilitiesDetail</Role>
      <ShortName>Derivatives - Schedule of Change in Fair Value of Derivative Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2434402 - Disclosure - Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesAnalysisOfAmortizedCostAndEstimatedFairValuesOfSecuritiesAvailableForSaleDetail</Role>
      <ShortName>Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2434403 - Disclosure - Investment Securities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesAdditionalInformationDetail</Role>
      <ShortName>Investment Securities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2434404 - Disclosure - Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesAmortizedCostAndEstimatedFairValueOfDebtSecuritiesAvailableForSaleDetail</Role>
      <ShortName>Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2434405 - Disclosure - Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale, Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesAmortizedCostAndEstimatedFairValueOfDebtSecuritiesAvailableForSaleAdditionalInformationDetail</Role>
      <ShortName>Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale, Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2434406 - Disclosure - Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesProceedsFromSaleGrossGainsLossesRealizedOnSalesMaturitiesAndOtherThanTemporaryImpairmentChargesRelatedToSecuritiesAvailableForSaleDetail</Role>
      <ShortName>Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2434407 - Disclosure - Investment Securities - Amortized Cost and Fair Values of Debt Securities Held to Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesAmortizedCostAndFairValuesOfDebtSecuritiesHeldToMaturityDetails</Role>
      <ShortName>Investment Securities - Amortized Cost and Fair Values of Debt Securities Held to Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2434408 - Disclosure - Investment Securities - Schedule of Debt Securities Expected Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesScheduleOfDebtSecuritiesExpectedMaturityDetails</Role>
      <ShortName>Investment Securities - Schedule of Debt Securities Expected Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2435402 - Disclosure - Impairment of Investment Securities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecuritiesAdditionalInformationDetail</Role>
      <ShortName>Impairment of Investment Securities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2435403 - Disclosure - Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecuritiesScheduleOfUnrealizedLossesAndEstimatedFairValuesDetail</Role>
      <ShortName>Impairment of Investment Securities - Schedule of Unrealized Losses and Estimated Fair Values (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2435404 - Disclosure - Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecuritiesPooledTrustPreferredCollateralizedDebtObligationsDetail</Role>
      <ShortName>Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2435405 - Disclosure - Impairment of Investment Securities - Cumulative Roll Forward of Credit Losses Recognized in Earnings for Debt Securities Held and Not Intended to be Sold (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ImpairmentOfInvestmentSecuritiesCumulativeRollForwardOfCreditLossesRecognizedInEarningsForDebtSecuritiesHeldAndNotIntendedToBeSoldDetail</Role>
      <ShortName>Impairment of Investment Securities - Cumulative Roll Forward of Credit Losses Recognized in Earnings for Debt Securities Held and Not Intended to be Sold (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2440402 - Disclosure - Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesOutstandingBalancesOfLoanDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Outstanding Balances of Loan (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2440403 - Disclosure - Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesCreditRiskProfileByCreditworthinessDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Credit Risk Profile by Creditworthiness (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2440404 - Disclosure - Loans and Allowance for Credit Losses - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesAdditionalInformationDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2440405 - Disclosure - Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesAgeAnalysisOfPastDueLoansBySegmentDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Age Analysis of Past Due Loans by Segment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2440406 - Disclosure - Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesRecordedInvestmentAndUnpaidPrincipalBalanceForImpairedLoansWithAssociatedAllowanceDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Recorded Investment and Unpaid Principal Balance for Impaired Loans with Associated Allowance (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2440407 - Disclosure - Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesTroubledDebtRestructuredLoansAndCommitmentsDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2440408 - Disclosure - Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesTroubledDebtRestructuringsIdentifiedDuringPeriodDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Troubled Debt Restructurings Identified During Period (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2440409 - Disclosure - Loans and Allowance for Credit Losses - Restructured Loans that are Considered to be in Default (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesRestructuredLoansThatAreConsideredToBeInDefaultDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Restructured Loans that are Considered to be in Default (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2440410 - Disclosure - Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansAndAllowanceForCreditLossesAllowanceForCreditLossesDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2441402 - Disclosure - Commitments and Letters of Credit - Notional Amount of Commitments and Letter of Credit Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsAndLettersOfCreditNotionalAmountOfCommitmentsAndLetterOfCreditInstrumentsDetail</Role>
      <ShortName>Commitments and Letters of Credit - Notional Amount of Commitments and Letter of Credit Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>92</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2441403 - Disclosure - Commitments and Letters of Credit - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsAndLettersOfCreditAdditionalInformationDetail</Role>
      <ShortName>Commitments and Letters of Credit - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2444402 - Disclosure - Premises and Equipment - Schedule of Premises and Equipment (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/PremisesAndEquipmentScheduleOfPremisesAndEquipmentDetail</Role>
      <ShortName>Premises and Equipment - Schedule of Premises and Equipment (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2444403 - Disclosure - Premises and Equipment - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/PremisesAndEquipmentAdditionalInformationDetail</Role>
      <ShortName>Premises and Equipment - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2444404 - Disclosure - Premises and Equipment - Schedule of Future Minimum Rentals (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/PremisesAndEquipmentScheduleOfFutureMinimumRentalsDetail</Role>
      <ShortName>Premises and Equipment - Schedule of Future Minimum Rentals (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2445402 - Disclosure - Goodwill and Other Amortizing Intangible Assets - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/GoodwillAndOtherAmortizingIntangibleAssetsAdditionalInformationDetail</Role>
      <ShortName>Goodwill and Other Amortizing Intangible Assets - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2445403 - Disclosure - Goodwill and Other Amortizing Intangible Assets - Summarized Other Intangible Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/GoodwillAndOtherAmortizingIntangibleAssetsSummarizedOtherIntangibleAssetsDetail</Role>
      <ShortName>Goodwill and Other Amortizing Intangible Assets - Summarized Other Intangible Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2445404 - Disclosure - Goodwill and Other Amortizing Intangible Assets - Estimated Amortization Expense of Core Deposit Intangibles (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/GoodwillAndOtherAmortizingIntangibleAssetsEstimatedAmortizationExpenseOfCoreDepositIntangiblesDetail</Role>
      <ShortName>Goodwill and Other Amortizing Intangible Assets - Estimated Amortization Expense of Core Deposit Intangibles (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2446402 - Disclosure - Interest-Bearing Deposits - Components of Interest-Bearing Deposits (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InterestBearingDepositsComponentsOfInterestBearingDepositsDetail</Role>
      <ShortName>Interest-Bearing Deposits - Components of Interest-Bearing Deposits (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2446403 - Disclosure - Interest-Bearing Deposits - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InterestBearingDepositsAdditionalInformationDetail</Role>
      <ShortName>Interest-Bearing Deposits - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2446404 - Disclosure - Interest-Bearing Deposits - Scheduled Maturities of Time Deposits (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InterestBearingDepositsScheduledMaturitiesOfTimeDepositsDetail</Role>
      <ShortName>Interest-Bearing Deposits - Scheduled Maturities of Time Deposits (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2447402 - Disclosure - Short-term Borrowings - Summary of Short-Term Borrowings (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ShortTermBorrowingsSummaryOfShortTermBorrowingsDetail</Role>
      <ShortName>Short-term Borrowings - Summary of Short-Term Borrowings (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>2447403 - Disclosure - Short-term Borrowings - Interest Expense on Short-Term Borrowings (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ShortTermBorrowingsInterestExpenseOnShortTermBorrowingsDetail</Role>
      <ShortName>Short-term Borrowings - Interest Expense on Short-Term Borrowings (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>104</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>2448402 - Disclosure - Subordinated Debentures - Subordinated Debentures Outstanding (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesSubordinatedDebenturesOutstandingDetail</Role>
      <ShortName>Subordinated Debentures - Subordinated Debentures Outstanding (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>105</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>2448403 - Disclosure - Subordinated Debentures - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesAdditionalInformationDetail</Role>
      <ShortName>Subordinated Debentures - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>2449402 - Disclosure - Other Long-term Debt (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/OtherLongTermDebtDetail</Role>
      <ShortName>Other Long-term Debt (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/OtherLongTermDebtTables</ParentRole>
      <Position>107</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>2450402 - Disclosure - Fair Values of Assets and Liabilities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesAdditionalInformationDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>108</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>2450403 - Disclosure - Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesQuantitativeInputsAndAssumptionsUsedInLevel3FairValueMeasurementsDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>2450404 - Disclosure - Fair Values of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesScheduleOfAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>110</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>2450405 - Disclosure - Fair Values of Assets and Liabilities - Schedule of Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesScheduleOfChangesInLevel3AssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Schedule of Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>111</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>2450406 - Disclosure - Fair Values of Assets and Liabilities - Schedule of Assets Measured on Nonrecurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesScheduleOfAssetsMeasuredOnNonrecurringBasisDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Schedule of Assets Measured on Nonrecurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>2450407 - Disclosure - Fair Values of Assets and Liabilities - Carrying Amounts and Fair Values of Financial Instruments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesOfAssetsAndLiabilitiesCarryingAmountsAndFairValuesOfFinancialInstrumentsDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Carrying Amounts and Fair Values of Financial Instruments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>113</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>2451402 - Disclosure - Income Taxes - Schedule of Components of Income Tax Provision (Benefit) (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncomeTaxesScheduleOfComponentsOfIncomeTaxProvisionBenefitDetail</Role>
      <ShortName>Income Taxes - Schedule of Components of Income Tax Provision (Benefit) (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>114</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>2451403 - Disclosure - Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncomeTaxesScheduleOfEffectiveIncomeTaxRateReconciliationDetail</Role>
      <ShortName>Income Taxes - Schedule of Effective Income Tax Rate Reconciliation (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>115</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>2451404 - Disclosure - Income Taxes - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncomeTaxesAdditionalInformationDetail</Role>
      <ShortName>Income Taxes - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>116</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>2451405 - Disclosure - Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncomeTaxesScheduleOfDeferredTaxAssetsAndLiabilitiesDetail</Role>
      <ShortName>Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>117</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>2453402 - Disclosure - Retirement Plans - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansAdditionalInformationDetail</Role>
      <ShortName>Retirement Plans - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>118</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>2453403 - Disclosure - Retirement Plans - Schedule of Components of Net Periodic Benefit Cost (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfComponentsOfNetPeriodicBenefitCostDetail</Role>
      <ShortName>Retirement Plans - Schedule of Components of Net Periodic Benefit Cost (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>119</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>2453404 - Disclosure - Retirement Plans - Schedule of Changes in Benefit Obligations and Plan Assets (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfChangesInBenefitObligationsAndPlanAssetsDetail</Role>
      <ShortName>Retirement Plans - Schedule of Changes in Benefit Obligations and Plan Assets (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>120</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>2453405 - Disclosure - Retirement Plans - Schedule of Amounts Recognized as Other Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfAmountsRecognizedAsOtherLiabilitiesDetail</Role>
      <ShortName>Retirement Plans - Schedule of Amounts Recognized as Other Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>121</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>2453406 - Disclosure - Retirement Plans - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfAmountsRecognizedInAccumulatedOtherComprehensiveIncomeDetail</Role>
      <ShortName>Retirement Plans - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>122</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R123.htm</HtmlFileName>
      <LongName>2453407 - Disclosure - Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Benefit Obligation (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfWeightedAverageAssumptionsUsedToDetermineBenefitObligationDetail</Role>
      <ShortName>Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Benefit Obligation (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>123</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R124.htm</HtmlFileName>
      <LongName>2453408 - Disclosure - Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Cost (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfWeightedAverageAssumptionsUsedToDetermineNetPeriodicCostDetail</Role>
      <ShortName>Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Cost (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>124</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R125.htm</HtmlFileName>
      <LongName>2453409 - Disclosure - Retirement Plans - Schedule of Impact of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfImpactOfOnePercentagePointChangeInAssumedHealthCareCostTrendRatesDetail</Role>
      <ShortName>Retirement Plans - Schedule of Impact of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>125</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R126.htm</HtmlFileName>
      <LongName>2453410 - Disclosure - Retirement Plans - Schedule of Projected Benefit Payments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RetirementPlansScheduleOfProjectedBenefitPaymentsDetail</Role>
      <ShortName>Retirement Plans - Schedule of Projected Benefit Payments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>126</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R127.htm</HtmlFileName>
      <LongName>2454402 - Disclosure - Incentive Compensation Plan - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncentiveCompensationPlanAdditionalInformationDetail</Role>
      <ShortName>Incentive Compensation Plan - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>127</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R128.htm</HtmlFileName>
      <LongName>2454403 - Disclosure - Incentive Compensation Plan - Unvested Service-Based Restricted Stock Awards (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncentiveCompensationPlanUnvestedServiceBasedRestrictedStockAwardsDetail</Role>
      <ShortName>Incentive Compensation Plan - Unvested Service-Based Restricted Stock Awards (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>128</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R129.htm</HtmlFileName>
      <LongName>2454404 - Disclosure - Incentive Compensation Plan Incentive Compensation Plan - Restricted Stock Awards Estimated to be Granted on a Performance Award (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncentiveCompensationPlanIncentiveCompensationPlanRestrictedStockAwardsEstimatedToBeGrantedOnPerformanceAwardDetails</Role>
      <ShortName>Incentive Compensation Plan Incentive Compensation Plan - Restricted Stock Awards Estimated to be Granted on a Performance Award (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>129</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R130.htm</HtmlFileName>
      <LongName>2454405 - Disclosure - Incentive Compensation Plan - Unvested Target Award (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/IncentiveCompensationPlanUnvestedTargetAwardDetail</Role>
      <ShortName>Incentive Compensation Plan - Unvested Target Award (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>130</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R131.htm</HtmlFileName>
      <LongName>2455401 - Disclosure - Contingent Liabilities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/ContingentLiabilitiesAdditionalInformationDetail</Role>
      <ShortName>Contingent Liabilities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>131</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R132.htm</HtmlFileName>
      <LongName>2456402 - Disclosure - Related Party Transactions - Loans to Related Parties (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RelatedPartyTransactionsLoansToRelatedPartiesDetail</Role>
      <ShortName>Related Party Transactions - Loans to Related Parties (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>132</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R133.htm</HtmlFileName>
      <LongName>2457402 - Disclosure - Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RegulatoryRestrictionsAndCapitalAdequacyScheduleOfCapitalAmountAndRatioConsideredToBeCapitalizedDetail</Role>
      <ShortName>Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>133</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R134.htm</HtmlFileName>
      <LongName>2458401 - Disclosure - Capital - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CapitalAdditionalInformationDetail</Role>
      <ShortName>Capital - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>134</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R135.htm</HtmlFileName>
      <LongName>2459402 - Disclosure - Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Financial Condition (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyStatementsOfFinancialConditionDetail</Role>
      <ShortName>Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Financial Condition (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyTables</ParentRole>
      <Position>135</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R136.htm</HtmlFileName>
      <LongName>2459403 - Disclosure - Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyStatementsOfOperationsDetail</Role>
      <ShortName>Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Statements of Operations (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyTables</ParentRole>
      <Position>136</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R137.htm</HtmlFileName>
      <LongName>2459404 - Disclosure - Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyStatementsOfCashFlowDetail</Role>
      <ShortName>Condensed Financial Information of First Commonwealth Financial Corporation (parent company only) - Statements of Cash Flow (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyTables</ParentRole>
      <Position>137</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R138.htm</HtmlFileName>
      <LongName>2459405 - Disclosure - Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyAdditionalInformationDetail</Role>
      <ShortName>Condensed Financial Information of First Commonwealth Financial Corporation (Parent Company Only) - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CondensedFinancialInformationOfFirstCommonwealthFinancialCorporationParentCompanyOnlyTables</ParentRole>
      <Position>138</Position>
    </Report>
    <Report instance="fcf-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R139.htm</HtmlFileName>
      <LongName>2460402 - Disclosure - Subsequent Event (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubsequentEventDetails</Role>
      <ShortName>Subsequent Event (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/SubsequentEventNotes</ParentRole>
      <Position>139</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>fcf-20171231.xml</File>
    <File>fcf-20171231.xsd</File>
    <File>fcf-20171231_cal.xml</File>
    <File>fcf-20171231_def.xml</File>
    <File>fcf-20171231_lab.xml</File>
    <File>fcf-20171231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2014-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
