| **Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail) $ in Thousands** | 3 Months Ended | 9 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                        |           |           |   |   |
| Financing Receivable, Modifications, Subsequent Default, Number of Contracts                                                |           |           | 3 | 3 |
| Financing Receivable, Modifications, Subsequent Default, Recorded Investment | $                                            |           |           | $ 108 | $ 18 |
| Troubled Debt Restructuring Number Of Contracts Ninety Days Past Due                                                        | 2              | 1              |   |   |
| Financing Receivable, Modification, Subsequent Default, Recorded Investment | $                                             | $ 105          | $ 5            |   |   |
| Residential real estate [Member]                                                                                            |           |           |   |   |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                                        |           |           |   |   |
| Financing Receivable, Modifications, Subsequent Default, Number of Contracts                                                |           |           | 3 | 3 |
| Financing Receivable, Modifications, Subsequent Default, Recorded Investment | $                                            |           |           | $ 108 | $ 18 |
| Troubled Debt Restructuring Number Of Contracts Ninety Days Past Due                                                        | 2              | 1              |   |   |
| Financing Receivable, Modification, Subsequent Default, Recorded Investment | $                                             | $ 105          | $ 5            |   |   |

| X                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Modification, Subsequent Default, Recorded Investment
+ References
No definition available.
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| 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
Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524
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| 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_FinancingReceivableRecordedInvestmentPastDueLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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