| **Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                                                         |           |   |
| Number of Contracts                                                                                                                                          | 2              | 2 |
| Recorded Investment                                                                                                                                          | $ 16           | $ 980 |
| Real estate construction [Member]                                                                                                                            |           |   |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                                                         |           |   |
| Number of Contracts                                                                                                                                          | 0              | 2 |
| Recorded Investment                                                                                                                                          | 0              | 980 |
| Loans to individuals [Member]                                                                                                                                |           |   |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                                                         |           |   |
| Number of Contracts                                                                                                                                          | 1              | 0 |
| Recorded Investment                                                                                                                                          | $ 6            | $ 0 |

| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Troubled debt restructuring number of contracts 90 days past due.
+ References
No definition available.
+ Details**Name:**fcf_TroubledDebtRestructuringNumberOfContractsNinetyDaysPastDue**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Troubled debt restructuring recorded investment ninety days past due.
+ References
No definition available.
+ Details**Name:**fcf_TroubledDebtRestructuringRecordedInvestmentNinetyDaysPastDue**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |