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Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Troubled Debt Restructurings [Line Items]    
Financing Receivable, Modifications, Recorded Investment $ 21,608us-gaap_FinancingReceivableModificationsRecordedInvestment $ 29,536us-gaap_FinancingReceivableModificationsRecordedInvestment
Unused lines of Credit [Member]    
Financing Receivable, Troubled Debt Restructurings [Line Items]    
Loans and Leases Receivable, Impaired, Commitment to Lend 391us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend
/ fcf_UnusedCommitmentsAxis
= us-gaap_UnusedLinesOfCreditMember		 4,120us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend
/ fcf_UnusedCommitmentsAxis
= us-gaap_UnusedLinesOfCreditMember
Accrual Loans [Member] [Member]    
Financing Receivable, Troubled Debt Restructurings [Line Items]    
Financing Receivable, Modifications, Recorded Investment 12,630us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_RestructuringCostAndReserveAxis
= fcf_AccrualLoansMember		 12,584us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_RestructuringCostAndReserveAxis
= fcf_AccrualLoansMember
Non Accrual Loans [Member]    
Financing Receivable, Troubled Debt Restructurings [Line Items]    
Financing Receivable, Modifications, Recorded Investment $ 8,978us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_RestructuringCostAndReserveAxis
= fcf_NonAccrualLoansMember		 $ 16,952us-gaap_FinancingReceivableModificationsRecordedInvestment
/ us-gaap_RestructuringCostAndReserveAxis
= fcf_NonAccrualLoansMember