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Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
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Loans and Leases Receivable Disclosure [Abstract] |  |  |
Accrual status | $ 13,790 | $ 14,139 | $ 12,584
Nonaccrual status | 11,569 | 12,360 | 16,952
Total | 25,359 | 26,499 | 29,536
Unused lines of credit | $ 358 | $ 3,252 | $ 4,120

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- Definition Financing receivable troubled debt restructured loans accrual status. + References No definition available. + Details
Name: | fcf_FinancingReceivableTroubledDebtRestructuredLoansAccrualStatus
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Financing receivable troubled debt restructured loans non-accrual status. + References No definition available. + Details
Name: | fcf_FinancingReceivableTroubledDebtRestructuredLoansNonaccrualStatus
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Troubled debt restructuring loans. + References No definition available. + Details
Name: | fcf_TroubledDebtRestructuringLoans
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The total amount of unused lines of credit associated with troubled debt restructured loans as of the reporting date. + References No definition available. + Details
Name: | fcf_UnusedLineOfCreditTroubledDebtRestructured
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_LoansAndLeasesReceivableDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration