| **Contingent Liabilities - Additional Information (Detail) (Pending Litigation [Member])**       | 1 Months Ended                               | 12 Months Ended                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------|----------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| **Loss Contingencies [Line Items]**                                                              |                                         |                                                                                                                                                                                                                                                                                                             |
| Number of new complaints                                                                         | 3us-gaap_LossContingencyNewClaimsFiledNumber |                                                                                                                                                                                                                                                                                                             |
| Number of former members of McGrogan class                                                       |                                         | 34fcf_LossContingencyFormerMembersofClassActionLawsuit                                                                                                                                                                                                                                                           |
| McGrogan Class Action [Member] | Ira Market Rate For Savings Account [Member]                    |                                         |                                                                                                                                                                                                                                                                                                             |
| **Loss Contingencies [Line Items]**                                                              |                                         |                                                                                                                                                                                                                                                                                                             |
| Interest rate on deposits                                                                        |                                         | 3.50%fcf_PercentageOfInterestRateOnDeposits/ us-gaap_FinancialInstrumentAxis= fcf_IraMarketRateForSavingsAccountMember/ us-gaap_LitigationCaseAxis= fcf_McGroganClassActionMember/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_PendingLitigationMember                                           |
| McGrogan Class Action [Member] | Minimum [Member] | Ira Market Rate For Savings Account [Member] |                                         |                                                                                                                                                                                                                                                                                                             |
| **Loss Contingencies [Line Items]**                                                              |                                         |                                                                                                                                                                                                                                                                                                             |
| Interest rate on deposits                                                                        |                                         | 8.00%fcf_PercentageOfInterestRateOnDeposits/ us-gaap_FinancialInstrumentAxis= fcf_IraMarketRateForSavingsAccountMember/ us-gaap_LitigationCaseAxis= fcf_McGroganClassActionMember/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_PendingLitigationMember/ us-gaap_RangeAxis= us-gaap_MinimumMember |

| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loss Contingency, Former Members of Class Action Lawsuit
+ References
No definition available.
+ Details**Name:**fcf_LossContingencyFormerMembersofClassActionLawsuit**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of interest rate on deposits.
+ References
No definition available.
+ Details**Name:**fcf_PercentageOfInterestRateOnDeposits**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total number of new claims filed pertaining to a loss contingency during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349
+ Details**Name:**us-gaap_LossContingencyNewClaimsFiledNumber**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LitigationCaseAxis=fcf_McGroganClassActionMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |