| **Premises and Equipment - Schedule of Future Minimum Rentals (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                           |
|--------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------|
| Premises [Member]                                                                                                        |                                                                                                                                    |
| **Operating Leased Assets [Line Items]**                                                                                 |                                                                                                                                    |
| 2014                                                                                                                     | $ 3,257fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueCurrent/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember     |
| 2015                                                                                                                     | 2,969fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInTwoYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember    |
| 2016                                                                                                                     | 2,758fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInThreeYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember  |
| 2017                                                                                                                     | 2,582fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInFourYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember   |
| 2018                                                                                                                     | 2,330fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInFiveYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember   |
| Therafter                                                                                                                | 12,276fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueThereafter/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember   |
| Total                                                                                                                    | 26,172fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDue/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= fcf_PremisesMember             |
| Equipment [Member]                                                                                                       |                                                                                                                                    |
| **Operating Leased Assets [Line Items]**                                                                                 |                                                                                                                                    |
| 2014                                                                                                                     | 56fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueCurrent/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember     |
| 2015                                                                                                                     | 0fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInTwoYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember   |
| 2016                                                                                                                     | 0fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInThreeYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember |
| 2017                                                                                                                     | 0fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInFourYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember  |
| 2018                                                                                                                     | 0fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueInFiveYears/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember  |
| Therafter                                                                                                                | 0fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDueThereafter/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember   |
| Total                                                                                                                    | $ 56fcf_FutureMinimumRentExpenseCorporateOfficeLeasesDue/ us-gaap_PropertyPlantAndEquipmentByTypeAxis= us-gaap_EquipmentMember          |

| X                                                                                                                                                                                                                                                                                            |
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Future Minimum Rent Expense Corporate Office Leases Due
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No definition available.
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                     |
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| - Definition
Future Minimum Rent Expense Corporate Office Leases Due In Five Years
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                     |
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| - Definition
Future Minimum Rent Expense Corporate Office Leases Due In Four Years
+ References
No definition available.
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| - Definition
Future Minimum Rent Expense Corporate Office Leases Due In Three Years
+ References
No definition available.
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| - Definition
Future Minimum Rent Expense Corporate Office Leases Due In Two Years
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                 |
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No definition available.
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| X                                                                                                                                                                  |
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| X                                                                                                                                                      |
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| X                                                                                                                                                           |
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