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v3.20.4

Retirement Plans - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Employer contribution | 6.00% |  |
Employee contribution | 80.00% |  |
Plan expense | $ 3,900 | $ 3,600 | $ 3,200
Retirement Plan expense | $ 0 | $ 0 | $ 0
Minimum [Member] |  |  |
Eligibility of participants | 1.00% |  |
Maximum [Member] |  |  |
Eligibility of participants | 25.00% |  |

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- Definition Deferred Compensation Plan Net Of Contribution Expenses + References No definition available. + Details
Name: | fcf_DeferredCompensationPlanNetOfContributionExpenses
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Participant Contribution On Pre Tax Basis Under Retirement Plan + References No definition available. + Details
Name: | fcf_ParticipantContributionOnPreTaxBasisUnderRetirementPlan
Namespace Prefix: | fcf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage Of Employees Eligible Compensation That Represents Maximum Amount Of Employer Matching Contributions To Four Zero One K Plan + References No definition available. + Details
Name: | fcf_PercentageOfEmployeesEligibleCompensationThatRepresentsMaximumAmountOfEmployerMatchingContributionsToFourZeroOneKPlan
Namespace Prefix: | fcf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Retirement Plan Expense + References No definition available. + Details
Name: | fcf_RetirementPlanExpense
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |