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Commitments and Contingent Liabilities - Additional Information (Detail) (USD $) | Mar. 31, 2015 | Dec. 31, 2014
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Loss Contingencies [Line Items] |  |
Notional Amount Of Performance Standby Letters Of Credit | $ 1,500,000 fcf_NotionalAmountOfPerformanceStandbyLettersOfCredit |
Notional Amount Of Commercial Letters Of Credit | 0 fcf_NotionalAmountOfCommercialLettersOfCredit |
Notional Amount Of Financial Standby Letters Of Credit | 0 fcf_NotionalAmountOfFinancialStandbyLettersOfCredit |
Financial instrument of credit risk | 200,000 us-gaap_FairValueConcentrationOfRiskCommitments | 200,000 us-gaap_FairValueConcentrationOfRiskCommitments
Unfunded commitment liability | $ 3,600,000 fcf_ReserveForOffBalanceSheetActivities | $ 3,100,000 fcf_ReserveForOffBalanceSheetActivities

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- Definition Notional amount of commercial letters of credit. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Notional Amount Of Financial Standby Letters Of Credit + References No definition available. + Details
Name: | fcf_NotionalAmountOfFinancialStandbyLettersOfCredit
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Notional amount of performance standby letters of credit. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Reserve for credit risk of off-balance sheet activities + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 17 -Article 9 + Details
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Balance Type: | credit
Period Type: | instant

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- Details
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