| **Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Fair Value Disclosures [Abstract]**                                                                                                                          |           |           |   |   |
| Impaired loans                                                                                                                                                 | $ (3,020)      | $ (10,806)     | $ (2,739) | $ (11,086) |
| Other real estate owned                                                                                                                                        | (728)          | (241)          | (1,078) | (362) |
| Total losses                                                                                                                                                   | $ (3,748)      | $ (11,047)     | $ (3,817) | $ (11,448) |

| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net losses on assets measured on nonrecurring basis.
+ References
No definition available.
+ Details**Name:**fcf_FairValueMeasurementOfAssetsOnNonrecurringBasisNetLoss**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net losses on impaired loans still held at the end of the period.
+ References
No definition available.
+ Details**Name:**fcf_FairValueMeasurementOfAssetsOnNonrecurringBasisNetLossesOnImpairedLoans**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Net losses on other real estate owned still held at the end of the period
+ References
No definition available.
+ Details**Name:**fcf_FairValueMeasurementOfAssetsOnNonrecurringBasisNetLossesOnOtherRealEstateOwned**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueDisclosuresAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |