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Premises and Equipment - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Property, Plant and Equipment [Line Items]      
Amount paid under sale-leaseback arrangement $ 321,000us-gaap_SaleLeasebackTransactionAmountDueUnderFinancingArrangement    
Gain on sale of branch 297,000fcf_GainOnSaleOfBranches    
Sale-leaseback transaction 15 year    
Total lease expense 2,900,000us-gaap_LeaseAndRentalExpense 4,200,000us-gaap_LeaseAndRentalExpense 4,300,000us-gaap_LeaseAndRentalExpense
Premises and Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Depreciation included in noninterest expense 13,200,000us-gaap_Depreciation
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= fcf_PremisesAndEquipmentMember
10,400,000us-gaap_Depreciation
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= fcf_PremisesAndEquipmentMember
7,900,000us-gaap_Depreciation
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= fcf_PremisesAndEquipmentMember
Software [Member]      
Property, Plant and Equipment [Line Items]      
Property Plant and Equipment, Gross, Amount Subject to Reduction in Estimated Useful Life 14,000,000fcf_PropertyPlantandEquipmentGrossAmountSubjecttoReductioninEstimatedUsefulLife
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_ComputerSoftwareIntangibleAssetMember
   
Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Property Plant and Equipment, Gross, Amount Subject to Reduction in Estimated Useful Life $ 700,000fcf_PropertyPlantandEquipmentGrossAmountSubjecttoReductioninEstimatedUsefulLife
/ us-gaap_PropertyPlantAndEquipmentByTypeAxis
= us-gaap_EquipmentMember
   
Previously Reported [Member] | Software [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Life 6 years    
Previously Reported [Member] | Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Life 5 years    
After System Conversion in 2014 [Member] | Software [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Life 3 years    
After System Conversion in 2014 [Member] | Equipment [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Life 4 years