Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Impaired loans	$ (7,850)	$ (1,460)
Other real estate owned	(30)	(31)
Total losses	$ (7,880)	$ (1,491)

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