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Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
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Mar. 31, 2015 Bank | Dec. 31, 2014
Impairment Of Investment Securities [Line Items] |  |
Securities available for sale, at fair value | $ 1,269,285 us-gaap_AvailableForSaleSecurities | $ 1,309,819 us-gaap_AvailableForSaleSecurities
Unrealized Gain (Loss) | 17,974 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax | 24,420 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 41,868 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
Securities available for sale, at fair value | 31,134 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
Unrealized Gain (Loss) | (10,734)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IV [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 1,830 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIvMember |
Securities available for sale, at fair value | 1,324 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIvMember |
Unrealized Gain (Loss) | (506)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIvMember |
Debt Instrument, Credit Rating | B1/BB |
Number of Banks (in banks) | 6 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIvMember |
Deferrals and Defaults as a % of Current Collateral | 18.05%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIvMember |
Excess Subordination as a % of Current Performing Collateral | 56.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIvMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 2,789 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiMember |
Securities available for sale, at fair value | 3,147 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiMember |
Unrealized Gain (Loss) | 358 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiMember |
Debt Instrument, Credit Rating | Ca/- |
Number of Banks (in banks) | 14 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiMember |
Deferrals and Defaults as a % of Current Collateral | 49.68%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiMember |
Excess Subordination as a % of Current Performing Collateral | 0.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL VIII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 1,984 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiiMember |
Securities available for sale, at fair value | 1,446 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiiMember |
Unrealized Gain (Loss) | (538)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiiMember |
Debt Instrument, Credit Rating | C/C |
Number of Banks (in banks) | 29 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiiMember |
Deferrals and Defaults as a % of Current Collateral | 58.01%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiiMember |
Excess Subordination as a % of Current Performing Collateral | 0.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslViiiMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL IX [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 2,349 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIxMember |
Securities available for sale, at fair value | 1,597 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIxMember |
Unrealized Gain (Loss) | (752)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIxMember |
Debt Instrument, Credit Rating | B3/C |
Number of Banks (in banks) | 40 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIxMember |
Deferrals and Defaults as a % of Current Collateral | 28.91%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIxMember |
Excess Subordination as a % of Current Performing Collateral | 6.61%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslIxMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL X [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 1,524 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXMember |
Securities available for sale, at fair value | 1,669 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXMember |
Unrealized Gain (Loss) | 145 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXMember |
Debt Instrument, Credit Rating | Caa1/C |
Number of Banks (in banks) | 43 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXMember |
Deferrals and Defaults as a % of Current Collateral | 31.60%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXMember |
Excess Subordination as a % of Current Performing Collateral | 0.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 5,559 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiMember |
Securities available for sale, at fair value | 3,969 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiMember |
Unrealized Gain (Loss) | (1,590)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiMember |
Debt Instrument, Credit Rating | B3/C |
Number of Banks (in banks) | 66 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiMember |
Deferrals and Defaults as a % of Current Collateral | 25.23%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiMember |
Excess Subordination as a % of Current Performing Collateral | 0.00%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIII [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 12,525 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiiMember |
Securities available for sale, at fair value | 8,870 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiiMember |
Unrealized Gain (Loss) | (3,655)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiiMember |
Debt Instrument, Credit Rating | B3/C |
Number of Banks (in banks) | 56 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiiMember |
Deferrals and Defaults as a % of Current Collateral | 25.44%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiiMember |
Excess Subordination as a % of Current Performing Collateral | 20.85%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXiiiMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | Pre TSL XIV [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 12,876 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXivMember |
Securities available for sale, at fair value | 8,702 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXivMember |
Unrealized Gain (Loss) | (4,174)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXivMember |
Debt Instrument, Credit Rating | Caa1/C |
Number of Banks (in banks) | 56 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXivMember |
Deferrals and Defaults as a % of Current Collateral | 25.46%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXivMember |
Excess Subordination as a % of Current Performing Collateral | 38.55%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_PreTslXivMember |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Mezzanine [Member] | MMCap I [Member] |  |
Impairment Of Investment Securities [Line Items] |  |
Book Value | 432 fcf_AvailableForSaleSecuritiesBookValue / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_MmcapIMember |
Securities available for sale, at fair value | 410 us-gaap_AvailableForSaleSecurities / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_MmcapIMember |
Unrealized Gain (Loss) | $ (22)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_MmcapIMember |
Debt Instrument, Credit Rating | Ca/C |
Number of Banks (in banks) | 10 fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_MmcapIMember |
Deferrals and Defaults as a % of Current Collateral | 56.11%fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_MmcapIMember |
Excess Subordination as a % of Current Performing Collateral | 24.92%fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral / us-gaap_DebtSecurityAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = fcf_MezzanineMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = fcf_MmcapIMember |

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