| **Derivatives - Additional Information (Detail) $ in Thousands**         | 3 Months Ended | 9 Months Ended |           |
|--------------------------------------------------------------------------|----------------|----------------|-----------|
||
| **Risk participation Agreements [Line Items]**                           |           |           |      |   |   |
| HedgedItemsOfCashFlowHedge                                               | $ 200,000      |           | $ 200,000 |   |   |
| Derivative Instruments Designated As Hedging Instruments Interest Income | $ 565          | $ 29           | $ 1,503   | $ 29 |   |
| Risk Participation Agreements Purchased [Domain]                         |           |           |      |   |   |
| **Risk participation Agreements [Line Items]**                           |           |           |      |   |   |
| Number of interest rate swaps | Derivatives                              | 15             |           | 15        |   |   |
| Risk Participation Agreements Sold [Domain]                              |           |           |      |   |   |
| **Risk participation Agreements [Line Items]**                           |           |           |      |   |   |
| Number of interest rate swaps | Derivatives                              | 3              |           | 3         |   |   |
| Designated as Hedging Instrument [Member]                                |           |           |      |   |   |
| **Risk participation Agreements [Line Items]**                           |           |           |      |   |   |
| Derivative, Notional Amount                                              | $ 200,000      |           | $ 200,000 |   | $ 100,000 |
| Three Year Maturity [Member] | Designated as Hedging Instrument [Member] |           |           |      |   |   |
| **Risk participation Agreements [Line Items]**                           |           |           |      |   |   |
| Derivative, Notional Amount                                              | 85,000         |           | 85,000    |   |   |
| Four Year Maturity [Member] | Designated as Hedging Instrument [Member]  |           |           |      |   |   |
| **Risk participation Agreements [Line Items]**                           |           |           |      |   |   |
| Derivative, Notional Amount                                              | $ 115,000      |           | $ 115,000 |   |   |

| 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
HedgedItemsOfCashFlowHedge
+ References
No definition available.
+ Details**Name:**fcf_HedgedItemsOfCashFlowHedge**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
[Line Items] for Risk participation Agreements [Table]
+ References
No definition available.
+ Details**Name:**fcf_RiskparticipationAgreementsLineItems**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B
+ Details**Name:**invest_DerivativeNotionalAmount**Namespace Prefix:**invest_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of credit risk derivative instruments held.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5579245-113959Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5580258-113959
+ Details**Name:**us-gaap_NumberOfCreditRiskDerivativesHeld**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_RiskParticipationAgreementsAxis=fcf_RiskParticipationAgreementsPurchasedDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_RiskParticipationAgreementsAxis=fcf_RiskParticipationAgreementsSoldDomain**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeByNatureAxis=fcf_ThreeYearMaturityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeByNatureAxis=fcf_FourYearMaturityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |