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Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
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Loans and Leases Receivable Disclosure [Abstract] |  |  |
Accrual status | $ 11,563 | $ 13,790 | $ 14,139
Nonaccrual status | 11,222 | 11,569 | 12,360
Total | 22,785 | 25,359 | 26,499
Letters of Credit Outstanding, Amount | 60 | 0 | 0
Unused lines of credit | $ 54 | $ 358 | $ 3,252

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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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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
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