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Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Investment Securities [Line Items] |  |
Amortized Cost | $ 1,317,318us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 1,350,466us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains | 16,921us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains | 15,565us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
Gross Unrealized Losses | (24,420)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss | (47,666)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
Estimated Fair Value of Available for Sale | 1,309,819us-gaap_AvailableForSaleSecurities | 1,318,365us-gaap_AvailableForSaleSecurities
US States and Political Subdivisions Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 27,058us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 80us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Gains | 362us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Gross Unrealized Losses | (43)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Estimated Fair Value of Available for Sale | 27,377us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 80us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 6,682us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember | 6,693us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Gains | 573us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember | 328us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Losses | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember
Estimated Fair Value of Available for Sale | 7,255us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember | 7,021us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_CorporateDebtSecuritiesMember
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 41,926us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember | 42,040us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Gross Unrealized Gains | 309us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Gross Unrealized Losses | (13,236)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember | (18,517)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Estimated Fair Value of Available for Sale | 28,999us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember | 23,523us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,315,898us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember | 1,349,046us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember
Gross Unrealized Gains | 16,921us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember | 15,565us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember
Gross Unrealized Losses | (24,420)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember | (47,666)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember
Estimated Fair Value of Available for Sale | 1,308,399us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember | 1,316,945us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_DebtSecuritiesMember
Mortgage - Backed Securities - Residential [Member] | US Government Agencies Debt Securities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 23,344us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 22,639us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Gross Unrealized Gains | 2,595us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 2,624us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Gross Unrealized Losses | (3)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | (59)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Estimated Fair Value of Available for Sale | 25,936us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 25,204us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Mortgage - Backed Securities - Residential [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 947,635us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 1,009,519us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Gross Unrealized Gains | 13,076us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 12,531us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Gross Unrealized Losses | (9,830)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | (27,163)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Estimated Fair Value of Available for Sale | 950,881us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember | 994,887us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember
Mortgage - Backed Securities - Commercial [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 72us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 104us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Gross Unrealized Gains | 2us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 1us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Gross Unrealized Losses | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Estimated Fair Value of Available for Sale | 74us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 105us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Other Government - Sponsored Enterprises [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 269,181us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember | 267,971us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember
Gross Unrealized Gains | 4us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember | 81us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember
Gross Unrealized Losses | (1,308)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember | (1,927)us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember
Estimated Fair Value of Available for Sale | 267,877us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember | 266,125us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByLiabilityClassAxis = us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_OtherGovernmentSponsoredEnterprisesMember
Equities [Member] |  |
Investment Securities [Line Items] |  |
Amortized Cost | 1,420us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 1,420us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Gross Unrealized Gains | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Gross Unrealized Losses | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Estimated Fair Value of Available for Sale | $ 1,420us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | $ 1,420us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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