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Retirement Plans - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employer contribution	6.00%
Employee contribution	80.00%
Plan expense	$ 3,200,000	$ 2,800,000	$ 2,500,000
Retirement Plan expense	$ 0	$ 0
Minimum [Member]
Eligibility of participants	1.00%
Maximum [Member]
Eligibility of participants	25.00%

X

- Definition

Deferred Compensation Plan Net Of Contribution Expenses

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No definition available.

+ Details

Name:	fcf_DeferredCompensationPlanNetOfContributionExpenses
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Participant Contribution On Pre Tax Basis Under Retirement Plan

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No definition available.

+ Details

Name:	fcf_ParticipantContributionOnPreTaxBasisUnderRetirementPlan
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage Of Employees Eligible Compensation That Represents Maximum Amount Of Employer Matching Contributions To Four Zero One K Plan

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No definition available.

+ Details

Name:	fcf_PercentageOfEmployeesEligibleCompensationThatRepresentsMaximumAmountOfEmployerMatchingContributionsToFourZeroOneKPlan
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Retirement Plan Expense

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No definition available.

+ Details

Name:	fcf_RetirementPlanExpense
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

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No definition available.

+ Details

Name:	us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

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