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Contingent Liabilities - Additional Information (Detail) (Pending Litigation [Member]) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2013 complaint | Dec. 31, 2014
Loss Contingencies [Line Items] |  |
Number of new complaints | 3us-gaap_LossContingencyNewClaimsFiledNumber |
Number of former members of McGrogan class |  | 34fcf_LossContingencyFormerMembersofClassActionLawsuit
McGrogan Class Action [Member] | Ira Market Rate For Savings Account [Member] |  |
Loss Contingencies [Line Items] |  |
Interest rate on deposits |  | 3.50%fcf_PercentageOfInterestRateOnDeposits / us-gaap_FinancialInstrumentAxis = fcf_IraMarketRateForSavingsAccountMember / us-gaap_LitigationCaseAxis = fcf_McGroganClassActionMember / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_PendingLitigationMember
McGrogan Class Action [Member] | Minimum [Member] | Ira Market Rate For Savings Account [Member] |  |
Loss Contingencies [Line Items] |  |
Interest rate on deposits |  | 8.00%fcf_PercentageOfInterestRateOnDeposits / us-gaap_FinancialInstrumentAxis = fcf_IraMarketRateForSavingsAccountMember / us-gaap_LitigationCaseAxis = fcf_McGroganClassActionMember / us-gaap_LossContingenciesByNatureOfContingencyAxis = us-gaap_PendingLitigationMember / us-gaap_RangeAxis = us-gaap_MinimumMember

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- Definition Loss Contingency, Former Members of Class Action Lawsuit + References No definition available. + Details
Name: | fcf_LossContingencyFormerMembersofClassActionLawsuit
Namespace Prefix: | fcf_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Percentage of interest rate on deposits. + References No definition available. + Details
Name: | fcf_PercentageOfInterestRateOnDeposits
Namespace Prefix: | fcf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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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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- Definition The total number of new claims filed pertaining to a loss contingency during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349 + Details
Name: | us-gaap_LossContingencyNewClaimsFiledNumber
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_LitigationCaseAxis=fcf_McGroganClassActionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |