Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash paid during the year for:
Interest	$ 18,943us-gaap_InterestPaid	$ 23,022us-gaap_InterestPaid	$ 31,597us-gaap_InterestPaid
Income taxes	10,700us-gaap_IncomeTaxesPaidNet	3,080us-gaap_IncomeTaxesPaidNet	11,641us-gaap_IncomeTaxesPaidNet
Non-cash investing and financing activities:
ESOP loan reductions	0	0	1,600
Loans transferred to other real estate owned and repossessed assets	5,061us-gaap_TransferOfOtherRealEstate	12,326us-gaap_TransferOfOtherRealEstate	4,979us-gaap_TransferOfOtherRealEstate
Other Real Estate Owned and Settled Out of Period	0fcf_TransferOfOtherRealEstateOwned	348fcf_TransferOfOtherRealEstateOwned	0fcf_TransferOfOtherRealEstateOwned
Fair value of loans transferred from held to maturity to available for sale	3,035	20,135	0
Gross (decrease) increase in market value adjustment to securities available for sale	24,601	(33,792)	(874)
Unsettled treasury stock repurchases	$ 0fcf_Unsettledtreasurystockrepurchases	$ 0fcf_Unsettledtreasurystockrepurchases	$ 1,222fcf_Unsettledtreasurystockrepurchases

Name:	fcf_CashPaidForInterestAndIncomeTaxesAbstract
Namespace Prefix:	fcf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Gross increase in market value adjustment to securities available for sale.

+ References

No definition available.

+ Details

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

- Definition

Value of other real estate transferred from the entity's loan portfolio in noncash transactions.

+ References

No definition available.

+ Details

Name:	fcf_TransferOfOtherRealEstateOwned
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Unsettled treasury stock repurchases

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of cash provided by the employer through contributions or dividends that the ESOP used in the period to repay debt and interest on a third-party loan made directly to the ESOP.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 40

-Section 25

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6418455&loc=d3e16545-113920

+ Details

Name:	us-gaap_EmployeeStockOwnershipPlanESOPDebtStructureDirectLoanEmployerCashPaymentsUsedForDebtService
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes.

+ References

+ Details

Name:	us-gaap_IncomeTaxesPaidNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of cash paid for interest during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

Name:	us-gaap_InterestPaid
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Value transferred out of real estate owned (REO) in noncash transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

+ Details

Name:	us-gaap_TransferOfOtherRealEstate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Value of loans and leases held in the entity's portfolio transferred to loans and leases held-for-sale in noncash transactions.

+ References

+ Details

Name:	us-gaap_TransferOfPortfolioLoansAndLeasesToHeldForSale1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration