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Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Loans and Leases Receivable Disclosure [Abstract] |  |  |
Accrual status | $ 8,512 | $ 7,542 | $ 8,757
Nonaccrual status | 14,740 | 6,037 | 11,761
Total | 23,252 | 13,579 | 20,518
Letters of Credit Outstanding, Amount | 60 | 60 | 60
Unused lines of credit | 11 | 163 | 1,027
Commitments Net | $ 71 | $ 223 | $ 1,087

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- Definition Total commitments. + References No definition available. + Details
Name: | fcf_CommitmentsNet
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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

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