| **Derivatives - Schedule of Changes in Fair Value of Derivative Assets and Liabilities (Detail) - USD ($) $ in Thousands** | 3 Months Ended | 6 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Derivative [Line Items]**                                                                                                |           |           |   |   |
| Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net                                             | $ (37)         | $ (531)        | $ (35) | $ (1,545) |
| Derivative Instruments Designated As Hedging Instruments Interest Income                                                   |           | 424            |   |   |
| Derivative, Net Hedge Ineffectiveness Gain (Loss)                                                                          | (5)            | 26             |   |   |
| Interest Rate Swap [Member]                                                                                                |           |           |   |   |
| **Derivative [Line Items]**                                                                                                |           |           |   |   |
| Derivative Instruments Designated As Hedging Instruments Interest Income                                                   | 136            |           | 385 | 854 |
| Derivative, Net Hedge Ineffectiveness Gain (Loss)                                                                          |           |           | 73 | 41 |
| Forward Contracts [Member]                                                                                                 |           |           |   |   |
| **Derivative [Line Items]**                                                                                                |           |           |   |   |
| Derivative, Net Hedge Ineffectiveness Gain (Loss)                                                                          | (17)           | 0              | 95 | 0 |
| Foreign Exchange [Member]                                                                                                  |           |           |   |   |
| **Derivative [Line Items]**                                                                                                |           |           |   |   |
| Derivative, Net Hedge Ineffectiveness Gain (Loss)                                                                          | $ (1)          | $ 4            | $ 1 | $ 7 |

| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Derivative Instruments Designated As Hedging Instruments Interest Income
+ References
No definition available.
+ Details**Name:**fcf_DerivativeInstrumentsDesignatedAsHedgingInstrumentsInterestIncome**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959
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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_DerivativeLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) from the ineffectiveness of the hedge.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
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