| **Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) - USD ($) $ in Thousands** | Mar. 31, 2022 | Dec. 31, 2021 |
|---------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Commitments to extend credit [Member]                                                                                     |          |          |
| **Loss Contingencies [Line Items]**                                                                                       |          |          |
| Financial instrument of credit risk                                                                                       | $ 2,376,865   | $ 2,353,991   |
| Financial standby letters of credit [Member]                                                                              |          |          |
| **Loss Contingencies [Line Items]**                                                                                       |          |          |
| Financial instrument of credit risk                                                                                       | 17,985        | 18,824        |
| Performance standby letters of credit [Member]                                                                            |          |          |
| **Loss Contingencies [Line Items]**                                                                                       |          |          |
| Financial instrument of credit risk                                                                                       | 17,584        | 10,663        |
| Commercial letters of credit [Member]                                                                                     |          |          |
| **Loss Contingencies [Line Items]**                                                                                       |          |          |
| Financial instrument of credit risk                                                                                       | $ 975         | $ 975         |

| X                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Notional amount of outstanding commitments and letters of credit.
+ References
No definition available.
+ Details**Name:**fcf_NotionalAmountOfCommitmentsAndLettersOfCredit**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_LossContingenciesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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