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Impairment of Investment Securities - Additional Information (Detail) (USD $) | 12 Months Ended
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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.

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- Definition Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 325 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6478792&loc=d3e63345-112809 + Details
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