Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value Disclosures [Abstract]
Impaired loans	$ (156)	$ 386	$ 160	$ (13,982)
Other real estate owned	(116)	(114)	(1,196)	(331)
Total losses	$ (272)	$ 272	$ (1,036)	$ (14,313)

Net losses on other real estate owned still held at the end of the period

No definition available.

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