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Cash and Due from Banks - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Cash and Due from Banks [Abstract]
Cash or balances held with the Federal Reserve Bank	$ 10.3	$ 5.1

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- Definition

Average Reserve Balances With Federal Reserve Bank

+ References

No definition available.

+ Details

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

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- References

No definition available.

+ Details

Name:	us-gaap_CashAndDueFromBanksAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration