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Impairment of Investment Securities - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Security
Bank
Dec. 31, 2013
Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Corporate/Mortgage-Backed Securities, Amortized cost $ 1,317,318,000us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 1,350,466,000us-gaap_AvailableForSaleSecuritiesAmortizedCost  
Estimated Fair Value of Available for Sale 1,309,819,000us-gaap_AvailableForSaleSecurities 1,318,365,000us-gaap_AvailableForSaleSecurities  
Number of banks and other financial institutions comprising the security 279fcf_NumberOfBanksAndOtherFinancialInstitutionsComprisingSecurity    
Number Of Pooled Securities with No Senior Class 4fcf_NumberOfPooledSecuritiesWithNoSeniorClass    
Number of Securities with No Excess Subordination 5fcf_NumberOfSecuritiesWithNoExcessSubordination    
Number of Securities with Excess Subordination 5fcf_NumberOfSecuritiesWithExcessSubordination    
Collateral issued by financial institutions 15,000,000,000fcf_AssetSizeOfFinancialInstitution    
Fair Value Input Coupon Rates 10.00%fcf_FairValueInputCouponRates    
Coupon interest rate 7.00%fcf_CouponInterestRate    
Estimate of future cash flows prepayment percentage in year three 40.00%fcf_EstimateOfFutureCashFlowsPercentageInYearOneandTwo    
Estimate of future cash flows prepayment percentage thereafter 2.00%fcf_EstimateOfFutureCashFlowsPrepaymentPercentageAfterYearTwo    
Prepayment rate 5.00%us-gaap_FairValueInputsPrepaymentRate    
Probability percentage assigned to default bank subject to market indicators 10.00%fcf_ProbabilityPercentageAssignedToDefaultBankSubjectToMarketIndicators    
Probability of default 100.00%fcf_ProbabilityOfDefault    
Probability percentage assigned to default bank 100.00%fcf_ProbabilityPercentageAssignedToDefaultBank    
Projected recovery rate 0.00%fcf_ProjectedRecoveryRate    
Net securities gains (losses) 550,000us-gaap_GainLossOnSaleOfSecuritiesNet (1,158,000)us-gaap_GainLossOnSaleOfSecuritiesNet 192,000us-gaap_GainLossOnSaleOfSecuritiesNet
Other investments 44,545,000us-gaap_FederalHomeLoanBankStock 35,444,000us-gaap_FederalHomeLoanBankStock  
Proceeds from the redemption of FHLB stock 31,819,000us-gaap_ProceedsFromSaleOfFederalReserveStock 10,904,000us-gaap_ProceedsFromSaleOfFederalReserveStock 11,568,000us-gaap_ProceedsFromSaleOfFederalReserveStock
Excess recognized as an adjustment to yield (1,297,000)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows [1] (2,375,000)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows [1] (1,462,000)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows [1]
Other than-temporary impairment charge 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold 13,356,000us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
Minimum [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Subordinated tranches range 7.00%fcf_SubordinatedTranchesRange
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Total principal amount of the respective securities 5.00%fcf_PercentageOfPrincipalAmountInPooledSecuritySecurities
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Excess subordination as a percentage of current performing collateral 7.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Collateral issued by financial institutions 15,000,000,000fcf_AssetSizeOfFinancialInstitution
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Probabilities for performing collateral range 0.33%fcf_ProbabilitiesForPerformingCollateralRange
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Excess percent value of future cash flows over current book value 22.00%fcf_ExcessPresentValueOfFutureCashFlowsOverOurCurrentBookValue
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Maximum [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Subordinated tranches range 35.00%fcf_SubordinatedTranchesRange
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Excess subordination as a percentage of current performing collateral 55.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Collateral issued by financial institutions 2,000,000,000fcf_AssetSizeOfFinancialInstitution
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Probabilities for performing collateral range 75.00%fcf_ProbabilitiesForPerformingCollateralRange
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Excess percent value of future cash flows over current book value 141.00%fcf_ExcessPresentValueOfFutureCashFlowsOverOurCurrentBookValue
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		    
Over fifteen billion [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Prepayment rate 100.00%us-gaap_FairValueInputsPrepaymentRate
/ us-gaap_RangeAxis
= fcf_OverFifteenMillionMember		    
Corporate Securities [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Corporate/Mortgage-Backed Securities, Amortized cost 6,682,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 6,693,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Estimated Fair Value of Available for Sale 7,255,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_CorporateDebtSecuritiesMember		 7,021,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Obligations of U.S. Government-Sponsored Enterprises [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Total unrealized losses 46.00%fcf_PercentageOfTotalUnrealizedLosses
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		    
Pooled Trust Preferred Collateralized Debt Obligations [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Total unrealized losses 54.00%fcf_PercentageOfTotalUnrealizedLosses
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		    
Corporate/Mortgage-Backed Securities, Amortized cost 41,926,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		 42,040,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		  
Estimated Fair Value of Available for Sale 28,999,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		 23,523,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		  
Mm Comm Ix [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Other than-temporary impairment charge 13,400,000us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_MmCommIxMember		    
Corporate Securities [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Corporate/Mortgage-Backed Securities, Amortized cost 6,600,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_CorporateSecuritiesMember		    
Net securities gains (losses) $ 1,300,000us-gaap_GainLossOnSaleOfSecuritiesNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_CorporateSecuritiesMember		    
Trust preferred collateralized debt obligations [Member]      
Schedule of Trading Securities and Other Trading Assets [Line Items]      
Percentage of Current Performing Collateral 0.00%fcf_PercentageOfCurrentPerformingCollateral
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_TrustPreferredCollateralizedDebtObligationsMember		    
[1] Represents the increase in cash flows recognized either as principal payments or interest income during the period.