| **Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) (USD $)In Thousands, unless otherwise specified** | Sep. 30, 2013 | Dec. 31, 2012 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Financing Receivable, Modifications, Recorded Investment                                                                                              | $ 29,097      | $ 64,016      |
| Loans and Leases Receivable, Impaired, Commitment to Lend                                                                                             | 3,244         | 1,574         |
| Letter of Credit [Member]                                                                                                                             |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Loans and Leases Receivable, Impaired, Commitment to Lend                                                                                             | 0             | 1,574         |
| Unused lines of Credit [Member]                                                                                                                       |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Loans and Leases Receivable, Impaired, Commitment to Lend                                                                                             | 3,244         | 0             |
| Non Accrual Loans [Member]                                                                                                                            |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Financing Receivable, Modifications, Recorded Investment                                                                                              | 11,290        | 13,037        |
| Non Accrual Loans [Member]                                                                                                                            |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Financing Receivable, Modifications, Recorded Investment                                                                                              | $ 17,807      | $ 50,979      |

| X                                                                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_FinancingReceivableTroubledDebtRestructuringsLineItemsLineItems**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6951948&loc=d3e18845-111554
+ Details**Name:**us-gaap_FinancingReceivableModificationsRecordedInvestment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reflects the unfunded portion of contractual agreements to lend funds to borrowers for loans whose terms have been modified by troubled debt restructuring.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 40 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6951948&loc=d3e18794-111554Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 15 -Paragraph 40 -Subparagraph b. -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |