| **Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) (USD $)In Thousands, unless otherwise specified** | Jun. 30, 2014 | Dec. 31, 2013 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Financing Receivable, Modifications, Recorded Investment                                                                                              | $ 17,359      | $ 30,475      |
| Loans and Leases Receivable, Impaired, Commitment to Lend                                                                                             | 2,482         | 452           |
| Letter of Credit [Member]                                                                                                                             |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Loans and Leases Receivable, Impaired, Commitment to Lend                                                                                             | 0             | 0             |
| Unused lines of Credit [Member]                                                                                                                       |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Loans and Leases Receivable, Impaired, Commitment to Lend                                                                                             | 2,482         | 452           |
| Non Accrual Loans [Member]                                                                                                                            |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Financing Receivable, Modifications, Recorded Investment                                                                                              | 10,566        | 13,495        |
| Non Accrual Loans [Member]                                                                                                                            |          |          |
| **Financing Receivable, Troubled Debt Restructurings [Line Items]**                                                                                   |          |          |
| Financing Receivable, Modifications, Recorded Investment                                                                                              | $ 6,793       | $ 16,980      |

| 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=25495931&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=25495931&loc=d3e18794-111554
+ Details**Name:**us-gaap_LoansAndLeasesReceivableImpairedCommitmentToLend**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |