| **Impairment of Investment Securities - Additional Information (Detail) (USD $)**                      | 3 Months Ended                                                                                                                                                                               |                                                                                                                                                                                              |                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **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                                 |   |

| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available-for-sale, securities in unrealized loss, percentage.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Excess present value of future cash flows over our current book value.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Excess subordination as a percentage of current performing collateral.
+ References
No definition available.
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| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs, Collateral
+ References
No definition available.
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| X                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs, Coupon Rate
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs , Prepayment Rate After Year Two
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Inputs , Prepayment Rate Year One and Two
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of banks and other financial institutions comprising the security.
+ References
No definition available.
+ Details**Name:**fcf_NumberOfBanksAndOtherFinancialInstitutionsComprisingSecurity**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Pooled Securities with No Senior Class
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities with Excess Subordination
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Securities with No Excess Subordination
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of principal amount in pooled security securities.
+ References
No definition available.
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| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percent of Unrealized Losses
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probabilities for performing collateral range.
+ References
No definition available.
+ Details**Name:**fcf_ProbabilitiesForPerformingCollateralRange**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probability of default.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probability percentage assigned to default bank.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Probability percentage assigned to default bank subject to market indicators.
+ References
No definition available.
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| X                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Projected recovery rate.
+ References
No definition available.
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| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Subordinated tranches range.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
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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
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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
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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
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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
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.
+ References
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