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Impairment of Investment Securities - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Sep. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other than temporary impairment charges recognized $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet $ 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet    
Amortized Cost 1,266,915,000us-gaap_AvailableForSaleSecuritiesAmortizedCost   1,317,318,000us-gaap_AvailableForSaleSecuritiesAmortizedCost  
Securities available for sale, at fair value 1,269,285,000us-gaap_AvailableForSaleSecurities   1,309,819,000us-gaap_AvailableForSaleSecurities  
Number of banks and other financial institutions comprising the security 278fcf_NumberOfBanksAndOtherFinancialInstitutionsComprisingSecurity      
Number Of Pooled Securities with No Senior Class 3fcf_NumberOfPooledSecuritiesWithNoSeniorClass      
Number of securities with no excess subordination 4fcf_NumberOfSecuritiesWithNoExcessSubordination      
Number of Securities with Excess Subordination 5fcf_NumberOfSecuritiesWithExcessSubordination      
Coupon rate 7.00%fcf_FairValueInputsCouponRate      
Probability of default 100.00%fcf_ProbabilityOfDefault      
Projected recovery rate 0.00%fcf_ProjectedRecoveryRate      
Probability percentage assigned to default bank subject to market indicators 10.00%fcf_ProbabilityPercentageAssignedToDefaultBankSubjectToMarketIndicators      
Probability percentage assigned to default bank 100.00%fcf_ProbabilityPercentageAssignedToDefaultBank      
Federal Home Loan Bank Stock 47,076,000us-gaap_FederalHomeLoanBankStock   44,545,000us-gaap_FederalHomeLoanBankStock  
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total 41fcf_InvestmentSecuritiesinUnrealizedLossPositionQualitativeDisclosureNumberofPositionsTotal      
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		      
Probabilities for performing collateral range 0.33%fcf_ProbabilitiesForPerformingCollateralRange
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Excess present value of future cash flows over our 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 56.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Probabilities for performing collateral range 75.00%fcf_ProbabilitiesForPerformingCollateralRange
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Excess present value of future cash flows over our current book value 147.00%fcf_ExcessPresentValueOfFutureCashFlowsOverOurCurrentBookValue
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Pooled Trust Preferred Collateralized Debt Obligations [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Percent of unrealized losses 62.00%fcf_PercentOfUnrealizedLosses
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		      
Amortized Cost 41,868,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		   41,926,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		  
Securities available for sale, at fair value 31,134,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		   28,999,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember		  
Trust preferred collateralized debt obligations [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other than temporary impairment charges recognized 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_TrustPreferredCollateralizedDebtObligationsMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_TrustPreferredCollateralizedDebtObligationsMember		    
Percentage of Current Performing Collateral 0.00%fcf_AvailableForSaleSecuritiesInUnrealizedLossPercentage
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_TrustPreferredCollateralizedDebtObligationsMember		      
Corporate Debt Securities [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Amortized Cost 6,680,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		   6,682,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Securities available for sale, at fair value 7,252,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		   7,255,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember		  
Pooled Trust Preferred Securities [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other than temporary impairment charges recognized 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_PooledTrustPreferredSecuritiesMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fcf_PooledTrustPreferredSecuritiesMember		    
Equity Securities [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Other than temporary impairment charges recognized 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 0us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		    
Continuous Unrealized Loss Position, Fair Value 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		     0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Amortized Cost 1,920,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		   1,420,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		  
Securities available for sale, at fair value 1,920,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		   1,420,000us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		  
Asset Size and Coupon Rate 1 [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Collateral issued by financial institutions 15,000,000,000fcf_FairValueInputsCollateral
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate1Member		      
Coupon rate 7.00%fcf_FairValueInputsCouponRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate1Member		      
Prepayment rate 100.00%us-gaap_FairValueInputsPrepaymentRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate1Member		      
Asset Size and Coupon Rate 2 [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Coupon rate 7.00%fcf_FairValueInputsCouponRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate2Member		      
Fair Value Inputs , Prepayment Rate Year One and Two 40.00%fcf_FairValueInputsPrepaymentRateYearOneAndTwo
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate2Member		      
Fair Value Inputs , Prepayment Rate After Year Two 2.00%fcf_FairValueInputsPrepaymentRateAfterYearTwo
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate2Member		      
Asset Size and Coupon Rate 3 [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Prepayment rate 5.00%us-gaap_FairValueInputsPrepaymentRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate3Member		      
Asset Size and Coupon Rate 3 [Member] | Minimum [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Collateral issued by financial institutions 2,000,000,000fcf_FairValueInputsCollateral
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate3Member
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		      
Asset Size and Coupon Rate 3 [Member] | Maximum [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Collateral issued by financial institutions 15,000,000,000fcf_FairValueInputsCollateral
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate3Member
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		      
Asset Size and Coupon Rate 4 [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Collateral issued by financial institutions 2,000,000,000fcf_FairValueInputsCollateral
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate4Member		      
Coupon rate 10.00%fcf_FairValueInputsCouponRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate4Member		      
Prepayment rate 5.00%us-gaap_FairValueInputsPrepaymentRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate4Member		      
Asset Size and Coupon Rate 5 [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Prepayment rate 0.00%us-gaap_FairValueInputsPrepaymentRate
/ fcf_EstimateOfFutureCashFlowAxis
= fcf_AssetSizeAndCouponRate5Member		      
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Percent of unrealized losses 38.00%fcf_PercentOfUnrealizedLosses
/ us-gaap_FairValueByLiabilityClassAxis
= us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember		      
Reported Value Measurement [Member]        
Schedule of Trading Securities and Other Trading Assets [Line Items]        
Securities available for sale, at fair value 1,269,285,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		   1,309,819,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		  
Federal Home Loan Bank Stock $ 47,076,000us-gaap_FederalHomeLoanBankStock
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		   $ 44,545,000us-gaap_FederalHomeLoanBankStock
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember