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

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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