| **Loans and Allowance for Credit Losses - Additional Information (Detail) (USD $)**                             | 3 Months Ended                                                                                                                                                                                                                                                                       |                                                                                                                                                                                |
|-----------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Charge-offs                                                                                                     | $ 7,109,000us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs                                                                                                                                                                                                              | $ 3,646,000us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs                                                                                                        |   |
| Unfunded commitments related to nonperforming loans                                                             | 600,000fcf_UnfundedCommitmentsRelatedToNonperformingLoans                                                                                                                                                                                                                            |                                                                                                                                                                           | 0fcf_UnfundedCommitmentsRelatedToNonperformingLoans |
| Off balance sheet reserve to nonperforming loans                                                                | 0fcf_OffBalanceSheetReserveToNonperformingLoans                                                                                                                                                                                                                                      |                                                                                                                                                                           |   |
| Loans held for sale                                                                                             | 3,000,000fcf_LoansHeldforSaleImpaired                                                                                                                                                                                                                                                |                                                                                                                                                                           | 0fcf_LoansHeldforSaleImpaired |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales                  | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales                                                                                                                                                                                            | 716,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales                                                                                |   |
| Total gains or losses included in earnings                                                                      | 105,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1                                                                                                                                                                | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1                                                                |   |
| Nonaccrual                                                                                                      | 33,565,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus                                                                                                                                                                                                              |                                                                                                                                                                           | 42,667,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus |
| Recorded investment                                                                                             | 46,195,000us-gaap_ImpairedFinancingReceivableRecordedInvestment                                                                                                                                                                                                                      |                                                                                                                                                                           | 55,251,000us-gaap_ImpairedFinancingReceivableRecordedInvestment |
| Loans to individuals [Member]                                                                                   |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Non Accrual Status Of Loans After Number Of Days Past Due                                                       | 150 days                                                                                                                                                                                                                                                                             |                                                                                                                                                                           |   |
| Impaired Loans [Member]                                                                                         |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Loans with modifications to rate and payment due to reamortization                                              | 300,000fcf_LoansWithModificationsToRateAndPaymentDueToReamortization/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                                                                                                                       | 200,000fcf_LoansWithModificationsToRateAndPaymentDueToReamortization/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= fcf_ImpairedLoansMember                                 |   |
| Loans Held For Sale [Member]                                                                                    |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales                  |                                                                                                                                                                                                                                                                                 | 716,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember                 |   |
| Total gains or losses included in earnings                                                                      |                                                                                                                                                                                                                                                                                 | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |   |
| Water Facility Construction [Member] | Nonaccrual Commercial Loans [Member]                                     |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Recorded investment                                                                                             | 9,300,000us-gaap_ImpairedFinancingReceivableRecordedInvestment/ fcf_LoanToDifferentProjectAxis= fcf_NonaccrualCommercialLoansMember/ fcf_NonaccrualCommercialLoansAxis= fcf_WaterFacilityConstructionMember                                                                          |                                                                                                                                                                           |   |
| Non Accrual Loans [Member] | Local Energy Company [Member] [Member]                                             |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Nonaccrual                                                                                                      | 6,100,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ fcf_LoanToDifferentProjectAxis= fcf_LocalEnergyCompanyMemberMember/ fcf_NonaccrualCommercialLoansAxis= fcf_NonAccrualLoansMember                                                                             |                                                                                                                                                                           |   |
| Charge-off recognized                                                                                           | 3,300,000fcf_Chargeoffrecognized/ fcf_LoanToDifferentProjectAxis= fcf_LocalEnergyCompanyMemberMember/ fcf_NonaccrualCommercialLoansAxis= fcf_NonAccrualLoansMember                                                                                                                   |                                                                                                                                                                           |   |
| 2012 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Nonaccrual                                                                                                      | 2,400,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ fcf_LoanToDifferentProjectAxis= fcf_LocalEnergyCompanyMemberMember/ fcf_NonaccrualCommercialLoansAxis= fcf_NonAccrualLoansMember/ fcf_PartofLoanRelationshipAxis= fcf_A2012TroubledDebtRestructuringMember   |                                                                                                                                                                           |   |
| 2014 Troubled Debt Restructuring [Member] | Non Accrual Loans [Member] | Local Energy Company [Member] [Member] |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| **Accounts, Notes, Loans and Financing Receivable [Line Items]**                                                |                                                                                                                                                                                                                                                                                 |                                                                                                                                                                           |   |
| Nonaccrual                                                                                                      | $ 3,100,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus/ fcf_LoanToDifferentProjectAxis= fcf_LocalEnergyCompanyMemberMember/ fcf_NonaccrualCommercialLoansAxis= fcf_NonAccrualLoansMember/ fcf_PartofLoanRelationshipAxis= fcf_A2014TroubledDebtRestructuringMember |                                                                                                                                                                           |   |

| X                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Charge-off recognized
+ References
No definition available.
+ Details**Name:**fcf_Chargeoffrecognized**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans Held for Sale, Impaired
+ References
No definition available.
+ Details**Name:**fcf_LoansHeldforSaleImpaired**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans with modifications to rate and payment due to reamortization.
+ References
No definition available.
+ Details**Name:**fcf_LoansWithModificationsToRateAndPaymentDueToReamortization**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Non Accrual Status Of Loans After Number Of Days Past Due
+ References
No definition available.
+ Details**Name:**fcf_NonAccrualStatusOfLoansAfterNumberOfDaysPastDue**Namespace Prefix:**fcf_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Off balance sheet reserve to nonperforming loans.
+ References
No definition available.
+ Details**Name:**fcf_OffBalanceSheetReserveToNonperformingLoans**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unfunded commitments related to nonperforming loans.
+ References
No definition available.
+ Details**Name:**fcf_UnfundedCommitmentsRelatedToNonperformingLoans**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AccountsNotesAndLoansReceivableLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of sale of 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5111-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953550-111524
+ Details**Name:**us-gaap_ImpairedFinancingReceivableRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_LoanToDifferentProjectAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_NonaccrualCommercialLoansAxis=fcf_NonAccrualLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_NonaccrualCommercialLoansAxis=fcf_WaterFacilityConstructionMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_PartofLoanRelationshipAxis=fcf_A2012TroubledDebtRestructuringMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_PartofLoanRelationshipAxis=fcf_A2014TroubledDebtRestructuringMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByAssetClassAxis=fcf_LoansHeldForSaleMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fcf_ImpairedLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |