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Derivatives - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012 Contract | Dec. 31, 2011 Contract
Increase in other income | $ 1.4 |
Number of interest rate swaps | 1 | 2
Credit risk expense |  | 6.7
Offset of credit risk |  | $ 4.4

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- Definition Derivative Credit Risk Expense (Income) + References No definition available. + Details
Name: | fcf_DerivativeCreditRiskExpenseIncome
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of increase in Other Income caused by reduction in credit risk on interest rate swaps. + References No definition available. + Details
Name: | fcf_IncreaseInOtherIncome
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Portion of the change in credit risk for the period. + References No definition available. + Details
Name: | fcf_OffsetOfCreditRisk
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Number of interest rate derivative instruments held by the entity at the reporting date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959 + Details
Name: | us-gaap_NumberOfInterestRateDerivativesHeld
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant