Fair Values of Assets and Liabilities - Losses Realized on Assets Measured on Non-Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Impaired loans	$ 205	$ (8,029)
Other real estate owned	(6)	(101)
Total losses	$ 199	$ (8,130)

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