| **Loans and Allowance for Credit Losses - Additional Information (Detail) (USD $)**            | 12 Months Ended                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Provision for credit losses                                                                    | $ 11,196,000us-gaap_ProvisionForLoanAndLeaseLosses                                                                                                                                                                                                 | $ 19,227,000us-gaap_ProvisionForLoanAndLeaseLosses | $ 20,544,000us-gaap_ProvisionForLoanAndLeaseLosses |
| Charge-offs                                                                                    | 17,472,000us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs                                                                                                                                                                             | 35,178,000us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | 16,969,000us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs |
| Recorded Investment                                                                            | 55,251,000us-gaap_ImpairedFinancingReceivableRecordedInvestment                                                                                                                                                                                    | 59,383,000us-gaap_ImpairedFinancingReceivableRecordedInvestment |   |
| Nonaccrual                                                                                     | 42,667,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                                                                            | 45,888,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus |   |
| Loans held for sale                                                                            | 2,502,000us-gaap_LoansReceivableHeldForSaleNet                                                                                                                                                                                                     | 0us-gaap_LoansReceivableHeldForSaleNet |   |
| Total gains or losses included in earnings                                                     | 77,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1                                                                                                                               | (770,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 |   |
| Loans with modification to rate and payment due to amortization                                | 600,000fcf_LoansWithModificationsToRateAndPaymentDueToReamortization                                                                                                                                                                               | 2,000,000fcf_LoansWithModificationsToRateAndPaymentDueToReamortization | 4,700,000fcf_LoansWithModificationsToRateAndPaymentDueToReamortization |
| Minimum days required for conversion of trouble debt to be in default                          | 90 days                                                                                                                                                                                                                                            |   |   |
| Water Treatment Plant [Member]                                                                 |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Nonaccrual loans                                                                               |                                                                                                                                                                                                                                               | 2,800,000fcf_NewNonAccrualLoans/ us-gaap_ParticipatingMortgageLoansAxis= fcf_WaterTreatmentPlantMember |   |
| Gas Well Servicing Operation [Member]                                                          |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Nonaccrual loans                                                                               |                                                                                                                                                                                                                                               | 3,700,000fcf_NewNonAccrualLoans/ us-gaap_ParticipatingMortgageLoansAxis= fcf_GasWellServicingOperationMember |   |
| Commercial real estate [Member]                                                                |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Nonaccrual loans                                                                               |                                                                                                                                                                                                                                               |   | 3,300,000fcf_NewNonAccrualLoans/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis= us-gaap_CommercialRealEstateMember |
| Loan maturity extension                                                                        |                                                                                                                                                                                                                                               |   | 6 months |
| Commercial real estate [Member] | Gas Well Servicing Operation [Member]                        |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Nonaccrual loans                                                                               |                                                                                                                                                                                                                                               | 17,000,000fcf_NewNonAccrualLoans/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis= us-gaap_CommercialRealEstateMember/ us-gaap_ParticipatingMortgageLoansAxis= fcf_GasWellServicingOperationMember |   |
| Impaired Loans [Member]                                                                        |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Increase In Commitments Related To Troubled Debt Restructured Loans                            | 3,700,000fcf_IncreaseInCommitmentsRelatedToTroubledDebtRestructuredLoans/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                                                                                 |   |   |
| Troubled Debt Restructured Loans Decreased                                                     | 900,000fcf_TroubledDebtRestructuredLoansDecreased/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                                                                                                        |   |   |
| Decrease in nonperforming loans                                                                | (4,100,000)fcf_IncreaseDecreaseInNonperformingLoans/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                                                                                                      |   |   |
| Decrease In Specific Allowance For Nonperforming Loans                                         | (600,000)fcf_DecreaseInSpecificAllowanceForNonperformingLoans/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                                                                                            |   |   |
| Unfunded commitments related to nonperforming loans                                            | 46,000fcf_UnfundedCommitmentsRelatedToNonperformingLoans/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                                                                                                 |   |   |
| Non Accrual Loans [Member]                                                                     |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Decrease in nonperforming loans                                                                | 32,200,000fcf_IncreaseDecreaseInNonperformingLoans/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_NonAccrualLoansMember                                                                                                                     |   |   |
| Unallocated [Member]                                                                           |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Provision for credit losses                                                                    |                                                                                                                                                                                                                                               | (5,926,000)us-gaap_ProvisionForLoanAndLeaseLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_UnallocatedFinancingReceivablesMember | 1,090,000us-gaap_ProvisionForLoanAndLeaseLosses/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_UnallocatedFinancingReceivablesMember |
| Charge-offs                                                                                    |                                                                                                                                                                                                                                               | 0us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_UnallocatedFinancingReceivablesMember | 0us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis= us-gaap_UnallocatedFinancingReceivablesMember |
| Consumer Loan [Member]                                                                         |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Non Accrual Status Of Loans After Number Of Days Past Due                                      | 150 days                                                                                                                                                                                                                                           |   |   |
| Nonaccrual Commercial Loans [Member]                                                           |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Amount Of Non Accrual Status Loan                                                              | 22,100,000fcf_AmountOfNonAccrualStatusLoan/ fcf_LoanToDifferentProjectAxis= fcf_NonaccrualCommercialLoansMember                                                                                                                                    |   |   |
| Nonaccrual Consumer Loans [Member]                                                             |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Amount Of Non Accrual Status Loan                                                              | 3,800,000fcf_AmountOfNonAccrualStatusLoan/ fcf_LoanToDifferentProjectAxis= fcf_NonaccrualConsumerLoansMember                                                                                                                                       |   |   |
| Western Pennsylvania Specialty Metal Processor [Member] | Nonaccrual Commercial Loans [Member] |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Amount Of Non Accrual Status Loan                                                              | 8,000,000fcf_AmountOfNonAccrualStatusLoan/ fcf_LoanToDifferentProjectAxis= fcf_NonaccrualCommercialLoansMember/ fcf_NonaccrualCommercialLoansAxis= fcf_WesternPennsylvaniaSpecialtyMetalProcessorMember                                            |   |   |
| Western Pennsylvania Reals Estate Manager [Member] | Nonaccrual Commercial Loans [Member]      |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Nonaccrual                                                                                     | 9,900,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ fcf_LoanToDifferentProjectAxis= fcf_NonaccrualCommercialLoansMember/ fcf_NonaccrualCommercialLoansAxis= fcf_WesternPennsylvaniaRealsEstateManagerMember                    |   |   |
| Local Energy Company [Member] | Nonaccrual Commercial Loans [Member]                           |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Nonaccrual                                                                                     | 10,000,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ fcf_LoanToDifferentProjectAxis= fcf_NonaccrualCommercialLoansMember/ fcf_NonaccrualCommercialLoansAxis= fcf_LocalEnergyCompanyMember                                      |   |   |
| Reserve for Off-balance Sheet Activities [Member] | Impaired Loans [Member]                    |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Off balance sheet reserve to nonperforming loans                                               | 0fcf_OffBalanceSheetReserveToNonperformingLoans/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis= us-gaap_ReserveForOffBalanceSheetActivitiesMember/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember |   |   |
| Loans Held for Sale [Member]                                                                   |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Total gains or losses included in earnings                                                     | $ 77,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember                                                              | $ 625,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |   |
| Commercial real estate [Member]                                                                |                                                                                                                                                                                                                                               |   |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                               |                                                                                                                                                                                                                                               |   |   |
| Financing Receivable, Modification, Subsequent Default, Number of Contracts                    |                                                                                                                                                                                                                                               | 1fcf_FinancingReceivableModificationSubsequentDefaultNumberofContracts/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis= us-gaap_CommercialRealEstateMember | 0fcf_FinancingReceivableModificationSubsequentDefaultNumberofContracts/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis= us-gaap_CommercialRealEstateMember |

| X                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of Non Accrual Status Loan
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| X                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Debt Instrument Maturity Extension Period
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| X                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Decrease in specific allowance for nonperforming loans.
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| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modification, Subsequent Default, Number of Contracts
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| X                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Increase (decrease) in nonperforming loans.
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| X                                                                                                                                                                                                                                                                                                                   |
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| - Definition
Increase In Commitments Related To Troubled Debt Restructured Loans
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| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans with modifications to rate and payment due to reamortization.
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| X                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
New loans moved into nonaccrual status.
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| X                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Non Accrual Status Of Loans After Number Of Days Past Due
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| X                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Off balance sheet reserve to nonperforming loans.
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| X                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Minimum days past due for TDR Default
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| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Troubled debt restructured loans decreased.
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| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unfunded commitments related to nonperforming loans.
+ References
No definition available.
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| X                                                                                                                                                                            |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount classified as loans attributable to disposal group held for sale or disposed of. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of expense related to estimated loss from loan and lease transactions.
+ References
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| X                                                                                                                                                          |
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| X                                                                                                                                                        |
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| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                         |
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| X                                                                                                                                                                              |
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| X                                                                                                                                                                                                              |
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| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
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