Cash and Due from Banks - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Cash and Due from Banks [Abstract]
Cash or balances held with the Federal Reserve Bank	$ 173.2	$ 14.7

Average Reserve Balances With Federal Reserve Bank

No definition available.

Name:	fcf_AverageReserveBalancesWithFederalReserveBank
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant