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Derivatives - Additional Information (Detail) | Dec. 31, 2014 Derivative
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Derivative Instruments and Hedging Activities Disclosure [Abstract] |
Number of participation agreements for interest rate swaps, participant | 13fcf_NumberofParticipationAgreementsInterestRateDerivativesHeldasaParticipant
Number of participation agreements for interest rate swaps, lead bank | 3us-gaap_NumberOfInterestRateDerivativesHeld

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- Definition Number of Participation Agreements, Interest Rate Derivatives Held as a Participant + References No definition available. + Details
Name: | fcf_NumberofParticipationAgreementsInterestRateDerivativesHeldasaParticipant
Namespace Prefix: | fcf_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of interest rate derivative instruments held by the entity at the reporting date. + References Reference 1: 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=35706170&loc=SL5580258-113959 Reference 2: 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=35706170&loc=SL5579245-113959 + Details
Name: | us-gaap_NumberOfInterestRateDerivativesHeld
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant