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Derivatives - Additional Information (Detail) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Risk participation Agreements [Line Items] |  |  |
HedgedItemsOfCashFlowHedge | $ 200,000,000 fcf_HedgedItemsOfCashFlowHedge |  |
Derivative Instruments Designated As Hedging Instruments Interest Income | 381,000 fcf_DerivativeInstrumentsDesignatedAsHedgingInstrumentsInterestIncome | 0 fcf_DerivativeInstrumentsDesignatedAsHedgingInstrumentsInterestIncome |
Risk Participation Agreements Purchased [Domain] |  |  |
Risk participation Agreements [Line Items] |  |  |
Number of interest rate swaps | 13 us-gaap_NumberOfCreditRiskDerivativesHeld / fcf_RiskParticipationAgreementsAxis = fcf_RiskParticipationAgreementsPurchasedDomain |  |
Risk Participation Agreements Sold [Domain] |  |  |
Risk participation Agreements [Line Items] |  |  |
Number of interest rate swaps | 3 us-gaap_NumberOfCreditRiskDerivativesHeld / fcf_RiskParticipationAgreementsAxis = fcf_RiskParticipationAgreementsSoldDomain |  |
Designated as Hedging Instrument [Member] |  |  |
Risk participation Agreements [Line Items] |  |  |
Derivative, Notional Amount | 200,000,000 invest_DerivativeNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  | 100,000,000 invest_DerivativeNotionalAmount / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember
Three Year Maturity [Member] | Designated as Hedging Instrument [Member] |  |  |
Risk participation Agreements [Line Items] |  |  |
Derivative, Notional Amount | 85,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = fcf_ThreeYearMaturityMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |
Four Year Maturity [Member] | Designated as Hedging Instrument [Member] |  |  |
Risk participation Agreements [Line Items] |  |  |
Derivative, Notional Amount | $ 115,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeByNatureAxis = fcf_FourYearMaturityMember / us-gaap_HedgingDesignationAxis = us-gaap_DesignatedAsHedgingInstrumentMember |  |

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- Definition Derivative Instruments Designated As Hedging Instruments Interest Income + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition HedgedItemsOfCashFlowHedge + References No definition available. + Details
Name: | fcf_HedgedItemsOfCashFlowHedge
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of credit risk derivative instruments held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
Name: | fcf_RiskParticipationAgreementsAxis=fcf_RiskParticipationAgreementsSoldDomain
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Data Type: | na
Balance Type: |
Period Type: |