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Retirement Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employer contribution	6.00%fcf_PercentageOfEmployeesEligibleCompensationThatRepresentsMaximumAmountOfEmployerMatchingContributionsToFourZeroOneKPlan
Employee contribution	80.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Plan expense	$ 2,600,000fcf_DeferredCompensationPlanNetOfContributionExpenses	$ 2,600,000fcf_DeferredCompensationPlanNetOfContributionExpenses	$ 2,600,000fcf_DeferredCompensationPlanNetOfContributionExpenses
Retirement Plan expense	0fcf_RetirementPlanExpense	0fcf_RetirementPlanExpense
Minimum [Member]
Total annual compensation for a calendar year for non-qualified deferred compensation plan	$ 110,000fcf_TotalAnnualCompensationforaCalendarYearforNonQualifiedDeferredCompensationPlan / us-gaap_RangeAxis = us-gaap_MinimumMember
Eligibility of participants	1.00%fcf_ParticipantContributionOnPreTaxBasisUnderRetirementPlan / us-gaap_RangeAxis = us-gaap_MinimumMember
Maximum [Member]
Eligibility of participants	25.00%fcf_ParticipantContributionOnPreTaxBasisUnderRetirementPlan / us-gaap_RangeAxis = us-gaap_MaximumMember

X

- 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

X

- 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

X

- 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

X

- 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

X

- Definition

Total Annual Compensation for a Calendar Year for Non-Qualified Deferred Compensation Plan

+ References

No definition available.

+ Details

Name:	fcf_TotalAnnualCompensationforaCalendarYearforNonQualifiedDeferredCompensationPlan
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.

+ References

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:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: