<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>718</ContextCount>
  <ElementCount>459</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>122</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>7</UnitCount>
  <MyReports>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0402415 - Document - Other Investments - Additional Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/OtherInvestmentsAdditionalInformation</Role>
      <ShortName>Other Investments - Additional Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001001 - Statement - Cover Page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CoverPage</Role>
      <ShortName>Cover Page</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002002 - Statement - Condensed Consolidated Statements of Financial Condition (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofFinancialConditionUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Financial Condition (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003003 - Statement - Condensed Consolidated Statements of Financial Condition (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofFinancialConditionUnauditedParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Financial Condition (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004004 - Statement - Condensed Consolidated Statements of Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofIncomeUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005005 - Statement - Condensed Consolidated Statements of Comprehensive Income (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofComprehensiveIncomeUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1006006 - Statement - Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofChangesinShareholdersEquityUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1007007 - Statement - Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofChangesinShareholdersEquityUnauditedParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>1008008 - Statement - Condensed Consolidated Statements of Cash Flows (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CondensedConsolidatedStatementsofCashFlowsUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>1109101 - Statement - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/BasisofPresentation</Role>
      <ShortName>Basis of Presentation</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>1110102 - Statement - Supplemental Comprehensive Income Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosures</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>1114103 - Statement - Supplemental Cash Flow Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalCashFlowDisclosures</Role>
      <ShortName>Supplemental Cash Flow Disclosures</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>1311301 - Statement - Supplemental Comprehensive Income Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosures_1</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>1315302 - Statement - Supplemental Cash Flow Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalCashFlowDisclosures_1</Role>
      <ShortName>Supplemental Cash Flow Disclosures</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>1412401 - Statement - Supplemental Comprehensive Income Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosures_2</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>1413402 - Statement - Supplemental Comprehensive Income Disclosures</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalComprehensiveIncomeDisclosures_3</Role>
      <ShortName>Supplemental Comprehensive Income Disclosures</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>1416403 - Statement - Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SupplementalCashFlowDisclosuresNoncashInvestingandFinancingActivities</Role>
      <ShortName>Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>1417444 - Statement - Revenue Recognition In-Scope and Out-of-Scope Revenue Streams</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RevenueRecognitionInScopeandOutofScopeRevenueStreams</Role>
      <ShortName>Revenue Recognition In-Scope and Out-of-Scope Revenue Streams</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2101104 - 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>19</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2105105 - Disclosure - Commitments and Contingent Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsandContingentLiabilities</Role>
      <ShortName>Commitments and Contingent Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2109106 - Disclosure - Investment Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecurities</Role>
      <ShortName>Investment Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2119108 - 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>22</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2123109 - 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>23</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2140112 - 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>24</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2154114 - Disclosure - Goodwill</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/Goodwill</Role>
      <ShortName>Goodwill</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2156115 - Disclosure - Subordinated Debentures Subordinated Debentures Outstanding</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesSubordinatedDebenturesOutstanding</Role>
      <ShortName>Subordinated Debentures Subordinated Debentures Outstanding</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2302303 - 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>27</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2306304 - Disclosure - Commitments and Contingent Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsandContingentLiabilitiesTables</Role>
      <ShortName>Commitments and Contingent Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/CommitmentsandContingentLiabilities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2310305 - 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>29</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2311306 - Disclosure - Investment Securities Maturity of Held-to-Maturity Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesMaturityofHeldtoMaturitySecuritiesTables</Role>
      <ShortName>Investment Securities Maturity of Held-to-Maturity Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2320307 - 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>31</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2324308 - 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>32</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2335309 - Disclosure - Leases Lease Liability Maturity (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LeasesLeaseLiabilityMaturityTables</Role>
      <ShortName>Leases Lease Liability Maturity (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2336310 - Disclosure - Leases Lease Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LeasesLeaseAssetsandLiabilitiesTables</Role>
      <ShortName>Leases Lease Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2341311 - 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>35</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2350312 - Disclosure - Derivatives (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/DerivativesTables</Role>
      <ShortName>Derivatives (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2357313 - Disclosure - Subordinated Debentures Subordinated Debentures Outstanding (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesSubordinatedDebenturesOutstandingTables</Role>
      <ShortName>Subordinated Debentures Subordinated Debentures Outstanding (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/SubordinatedDebenturesSubordinatedDebenturesOutstanding</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2361314 - Disclosure - Revenue Recognition Revenue Recogntion (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RevenueRecognitionRevenueRecogntionTables</Role>
      <ShortName>Revenue Recognition Revenue Recogntion (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2403404 - 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>39</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2404405 - 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>40</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2407406 - Disclosure - Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsandContingentLiabilitiesNotionalAmountofOutstandingCommitmentsDetail</Role>
      <ShortName>Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - Commitments and Contingent Liabilities - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/CommitmentsandContingentLiabilitiesAdditionalInformationDetail</Role>
      <ShortName>Commitments and Contingent Liabilities - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2412408 - 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>43</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2413409 - 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>44</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2414410 - Disclosure - Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (1)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesAmortizedCostandEstimatedFairValueofDebtSecuritiesAvailableforSaleDetail1</Role>
      <ShortName>Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Detail) (1)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2415411 - 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>46</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2416412 - 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>47</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2417413 - Disclosure - Investment Securities Schedule of Held-to-Maturity Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesScheduleofHeldtoMaturitySecuritiesDetails</Role>
      <ShortName>Investment Securities Schedule of Held-to-Maturity Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2418414 - Disclosure - Investment Securities Held to Maturity Securities by Type (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/InvestmentSecuritiesHeldtoMaturitySecuritiesbyTypeDetails</Role>
      <ShortName>Investment Securities Held to Maturity Securities by Type (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2421416 - 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>50</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2422417 - 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>51</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2425418 - 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>52</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2426419 - 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>53</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2427420 - 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>54</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2428421 - 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>55</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2429422 - 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>56</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2430423 - 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>57</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2431424 - 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>58</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2432425 - Disclosure - Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LoansandAllowanceforCreditLossesTroubledDebtRestructuringis90DaysorMorePastDueDetail</Role>
      <ShortName>Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2433426 - 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>60</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2437427 - Disclosure - Leases Operating Lease Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LeasesOperatingLeaseAssetsandLiabilitiesDetails</Role>
      <ShortName>Leases Operating Lease Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2438428 - Disclosure - Leases Lease Liablity Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LeasesLeaseLiablityMaturityDetails</Role>
      <ShortName>Leases Lease Liablity Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2439429 - Disclosure - Leases Topic 842 Additional Information (Details) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/LeasesTopic842AdditionalInformationDetailsDetails</Role>
      <ShortName>Leases Topic 842 Additional Information (Details) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2442430 - 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>64</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2443431 - 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>65</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2444432 - 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>66</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2445433 - 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>67</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2446434 - Disclosure - Fair Values of Assets and Liabilities - Schedule of Assets Measured on Non-Recurring 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 Non-Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2447435 - Disclosure - Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/FairValuesofAssetsandLiabilitiesLossesRealizedonAssetsMeasuredonNonRecurringBasisDetail</Role>
      <ShortName>Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2448436 - 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>70</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2451437 - 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>71</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2452438 - 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>72</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2453439 - Disclosure - Derivatives - Schedule of Changes in Fair Value of Derivative Assets and Liabilities (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/DerivativesScheduleofChangesinFairValueofDerivativeAssetsandLiabilitiesDetail</Role>
      <ShortName>Derivatives - Schedule of Changes in Fair Value of Derivative Assets and Liabilities (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2455440 - Disclosure - Goodwill - Additional Information (Detail)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/GoodwillAdditionalInformationDetail</Role>
      <ShortName>Goodwill - Additional Information (Detail)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2458441 - Disclosure - Subordinated Debentures Schedule of Subordinated Debentures Outstanding (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesScheduleofSubordinatedDebenturesOutstandingDetails</Role>
      <ShortName>Subordinated Debentures Schedule of Subordinated Debentures Outstanding (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2459442 - Disclosure - Subordinated Debentures Subordinated Debentures, Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/SubordinatedDebenturesSubordinatedDebenturesAdditionalInformationDetails</Role>
      <ShortName>Subordinated Debentures Subordinated Debentures, Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="fcf-20200331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2462443 - Disclosure - Revenue Recognition Revenue Recognition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.fcbanking.com/role/RevenueRecognitionRevenueRecognitionDetails</Role>
      <ShortName>Revenue Recognition Revenue Recognition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.fcbanking.com/role/RevenueRecognitionRevenueRecogntionTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="fcf-20200331.htm">fcf-20200331.htm</File>
    <File>fcf-20200331.xsd</File>
    <File>fcf-20200331_cal.xml</File>
    <File>fcf-20200331_def.xml</File>
    <File>fcf-20200331_lab.xml</File>
    <File>fcf-20200331_pre.xml</File>
    <File>fcf-20200331xexx101202.htm</File>
    <File>fcf-20200331xexx102202.htm</File>
    <File>fcf-ex31120200331x10q.htm</File>
    <File>fcf-ex31220200331x10q.htm</File>
    <File>fcf-ex32120200331x10q.htm</File>
    <File>fcf-ex32220200331x10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
