Derivatives - Schedule of Changes in Fair Value of Derivative Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ (230)	$ (58)
Derivative Instruments Designated As Hedging Instruments Interest Income	381	0
Derivative, Net Hedge Ineffectiveness Gain (Loss)	$ 9	$ 0

Derivative Instruments Designated As Hedging Instruments Interest Income

No definition available.

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

- Details

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

- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

+ Details

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

- Definition

Amount of gain (loss) from the ineffectiveness of the hedge.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4C

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 25

-Section 50

-Paragraph 1

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

+ Details

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