| **Derivatives (Tables)**                                                               | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Derivative Instruments and Hedging Activities Disclosure [Abstract]**                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
| Credit Value Adjustment Recorded Related to Notional Amount Of Derivatives Outstanding | The following table depicts the credit value adjustment recorded related to the notional amount of derivatives outstanding as well as the notional amount of risk participation agreements participated to other banks: September 30, 2015 December 31, 2014 (dollars in thousands)Derivatives not Designated as Hedging Instruments   Credit value adjustment$(688) $(268)Notional Amount:   Interest rate derivatives271,371 273,388Interest rate caps23,447 6,656Risk participation agreements124,116 113,624Sold credit protection on risk participation agreements(17,635) (17,296)Derivatives Designated as Hedging Instruments   Fair value adjustment2,644 472Notional Amount - Interest rate derivatives200,000 100,000 |
| Schedule of Changes in Fair Value of Derivative Assets and Liabilities                 | The table below presents the amount representing the change in the fair value of derivative assets and derivative liabilities attributable to credit risk included in Other income on the Condensed Consolidated Statements of Income: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2015 2014 2015 2014 (dollars in thousands)Non-hedging interest rate derivatives       (Decrease) increase in other income$(783) $(108) $(420) $175Hedging interest rate derivatives       Increase in interest and fees on loans565 29 1,503 29Increase in other expense— — 21 —                                                |

| X                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Notional amounts and credit value adjustment recorded related to the notional amount of derivatives outstanding.
+ References
No definition available.
+ Details**Name:**fcf_CreditValueAdjustmentRecordedRelatedToNotionalAmountOfDerivativesOutstandingTableTextBlock**Namespace Prefix:**fcf_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_DerivativeInstrumentsAndHedgingActivitiesDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of interest rate derivatives, including, but not limited to, the fair value of the derivatives, statement of financial position location, and statement of financial performance location of these instruments.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959
+ Details**Name:**us-gaap_ScheduleOfInterestRateDerivativesTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |