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Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2014 | Dec. 31, 2013
--- | --- | ---
Commitments to extend credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | $ 1,644,486 | $ 1,571,987
Financial standby letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | 29,379 | 38,121
Performance standby letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | 26,210 | 32,441
Commercial letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | $ 2,405 | $ 0

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- Definition Notional amount of outstanding commitments and letters of credit. + References No definition available. + Details
Name: | fcf_NotionalAmountOfCommitmentsAndLettersOfCredit
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration