| **Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) (USD $)In Thousands, unless otherwise specified** | Mar. 31, 2015                                                                                                                                             | Dec. 31, 2014                                                                                                                                             |
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| Commitments to extend credit [Member]                                                                                                            |                                                                                                                                                      |                                                                                                                                                      |
| **Loss Contingencies [Line Items]**                                                                                                              |                                                                                                                                                      |                                                                                                                                                      |
| Financial instrument of credit risk                                                                                                              | $ 1,645,977fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_CommitmentsToExtendCreditMember | $ 1,635,948fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_CommitmentsToExtendCreditMember |
| Financial standby letters of credit [Member]                                                                                                     |                                                                                                                                                      |                                                                                                                                                      |
| **Loss Contingencies [Line Items]**                                                                                                              |                                                                                                                                                      |                                                                                                                                                      |
| Financial instrument of credit risk                                                                                                              | 27,858fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_FinancialStandbyLetterOfCreditMember | 36,075fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_FinancialStandbyLetterOfCreditMember |
| Performance standby letters of credit [Member]                                                                                                   |                                                                                                                                                      |                                                                                                                                                      |
| **Loss Contingencies [Line Items]**                                                                                                              |                                                                                                                                                      |                                                                                                                                                      |
| Financial instrument of credit risk                                                                                                              | 26,490fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_PerformanceGuaranteeMember           | 25,915fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= us-gaap_PerformanceGuaranteeMember           |
| Commercial letters of credit [Member]                                                                                                            |                                                                                                                                                      |                                                                                                                                                      |
| **Loss Contingencies [Line Items]**                                                                                                              |                                                                                                                                                      |                                                                                                                                                      |
| Financial instrument of credit risk                                                                                                              | $ 2,584fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= fcf_CommercialLettersOfCreditMember         | $ 2,611fcf_NotionalAmountOfCommitmentsAndLettersOfCredit/ us-gaap_LossContingenciesByNatureOfContingencyAxis= fcf_CommercialLettersOfCreditMember         |

| X                                                                                                                                                                                                                                                                                                   |
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| - Definition
Notional amount of outstanding commitments and letters of credit.
+ References
No definition available.
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| - Details**Name:**us-gaap_LossContingenciesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                              |
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| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=fcf_CommercialLettersOfCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                  |
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| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_CommitmentsToExtendCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                       |
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| - Details**Name:**us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_FinancialStandbyLetterOfCreditMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                             |
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