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Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Commitments to extend credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | $ 1,645,977 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_CommitmentsToExtendCreditMember | $ 1,635,948 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_CommitmentsToExtendCreditMember
Financial standby letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | 27,858 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_FinancialStandbyLetterOfCreditMember | 36,075 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_FinancialStandbyLetterOfCreditMember
Performance standby letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | 26,490 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_PerformanceGuaranteeMember | 25,915 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_PerformanceGuaranteeMember
Commercial letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | $ 2,584 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = fcf_CommercialLettersOfCreditMember | $ 2,611 fcf_NotionalAmountOfCommitmentsAndLettersOfCredit / us-gaap_LossContingenciesByNatureOfContingencyAxis = fcf_CommercialLettersOfCreditMember

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- Definition Notional amount of outstanding commitments and letters of credit. + 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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- Details
Name: | us-gaap_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LossContingenciesByNatureOfContingencyAxis=us-gaap_PerformanceGuaranteeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |