| **Retirement Plans - Additional Information (Detail) (USD $)**                             | 12 Months Ended |
|--------------------------------------------------------------------------------------------|-----------------|
||
| Employer contribution                                                                      | 6.00%           |   |   |
| Employee contribution                                                                      | 80.00%          |   |   |
| Plan expense                                                                               | $ 2,600,000     | $ 2,600,000 | $ 2,500,000 |
| Retirement Plan expense                                                                    | 0               | 0 | 86,000 |
| Minimum [Member]                                                                           |            |   |   |
| Total annual compensation for a calendar year for non-qualified deferred compensation plan | $ 110,000       |   |   |
| Eligibility of participants                                                                | 1.00%           |   |   |
| Maximum [Member]                                                                           |            |   |   |
| Eligibility of participants                                                                | 25.00%          |   |   |

| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferred Compensation Plan Net Of Contribution Expenses
+ References
No definition available.
+ Details**Name:**fcf_DeferredCompensationPlanNetOfContributionExpenses**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Participant Contribution On Pre Tax Basis Under Retirement Plan
+ References
No definition available.
+ Details**Name:**fcf_ParticipantContributionOnPreTaxBasisUnderRetirementPlan**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage Of Employees Eligible Compensation That Represents Maximum Amount Of Employer Matching Contributions To Four Zero One K Plan
+ References
No definition available.
+ Details**Name:**fcf_PercentageOfEmployeesEligibleCompensationThatRepresentsMaximumAmountOfEmployerMatchingContributionsToFourZeroOneKPlan**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Retirement Plan Expense
+ References
No definition available.
+ Details**Name:**fcf_RetirementPlanExpense**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total Annual Compensation for a Calendar Year for Non-Qualified Deferred Compensation Plan
+ References
No definition available.
+ Details**Name:**fcf_TotalAnnualCompensationforaCalendarYearforNonQualifiedDeferredCompensationPlan**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Maximum percentage of employee gross pay, by the terms of the plan, that the employer may contribute to a defined contribution plan.
+ References
No definition available.
+ Details**Name:**us-gaap_DefinedContributionPlanMaximumAnnualContributionPerEmployeePercent**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |