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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 | $ 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet | $ 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet |  |
Amortized Cost | 1,266,915,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  | 1,317,318,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost |
Securities available for sale, at fair value | 1,269,285,000 us-gaap_AvailableForSaleSecurities |  | 1,309,819,000 us-gaap_AvailableForSaleSecurities |
Number of banks and other financial institutions comprising the security | 278 fcf_NumberOfBanksAndOtherFinancialInstitutionsComprisingSecurity |  |  |
Number Of Pooled Securities with No Senior Class | 3 fcf_NumberOfPooledSecuritiesWithNoSeniorClass |  |  |
Number of securities with no excess subordination | 4 fcf_NumberOfSecuritiesWithNoExcessSubordination |  |  |
Number of Securities with Excess Subordination | 5 fcf_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 | 47,076,000 us-gaap_FederalHomeLoanBankStock |  | 44,545,000 us-gaap_FederalHomeLoanBankStock |
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total | 41 fcf_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 | 41,868,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |  | 41,926,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
Securities available for sale, at fair value | 31,134,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |  | 28,999,000 us-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 | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_TrustPreferredCollateralizedDebtObligationsMember | 0 us-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 | 6,680,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 6,682,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Securities available for sale, at fair value | 7,252,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |  | 7,255,000 us-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 | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PooledTrustPreferredSecuritiesMember | 0 us-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 | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |
Continuous Unrealized Loss Position, Fair Value | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  |  | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember
Amortized Cost | 1,920,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 1,420,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |
Securities available for sale, at fair value | 1,920,000 us-gaap_AvailableForSaleSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_EquitySecuritiesMember |  | 1,420,000 us-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 | 15,000,000,000 fcf_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 | 2,000,000,000 fcf_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 | 15,000,000,000 fcf_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 | 2,000,000,000 fcf_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 | 1,269,285,000 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember |  | 1,309,819,000 us-gaap_AvailableForSaleSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember |
Federal Home Loan Bank Stock | $ 47,076,000 us-gaap_FederalHomeLoanBankStock / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember |  | $ 44,545,000 us-gaap_FederalHomeLoanBankStock / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember |

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- Definition Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total + References No definition available. + Details
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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=51813488&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=49167202&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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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563 + Details
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- Definition Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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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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