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Contingent Liabilities - Additional Information (Detail) (Pending Litigation [Member])
 1 Months Ended 12 Months Ended
Dec. 31, 2013
complaint
Dec. 31, 2014
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