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Unearned ESOP Shares - Interest Paid on ESOP Loan and Dividends Received on Unallocated Shares (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items] |  |  |
Interest paid on ESOP loan | $ 18,943us-gaap_InterestPaid | $ 23,022us-gaap_InterestPaid | $ 31,597us-gaap_InterestPaid
Dividends on unallocated shares |  |  | 19fcf_CashDividendsPaidOnUnallocatedEsopShares
ESOP [Member] |  |  |
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items] |  |  |
Interest paid on ESOP loan |  |  | $ 13us-gaap_InterestPaid / us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis = fcf_EmployeeStockOwnershipPlanMember

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- Definition Cash Dividends Paid On Unallocated Esop Shares + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Definition The amount of cash paid for interest during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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