| **Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended |           |
|----------------------------------------------------------------------------------------------------------------------------------|----------------|-----------|
||
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | $ 67,187       | $ 61,234  |   |
| Charge-offs                                                                                                                      | (10,476)       | (4,992)   |   |
| Recoveries                                                                                                                       | 1,054          | 703       |   |
| Provision                                                                                                                        | 4,497          | 3,787     |   |
| Ending Balance                                                                                                                   | 62,262         | 60,732    |   |
| Ending balance: individually evaluated for impaired                                                                              | 12,689         | 11,920    |   |
| Ending balance: collectively evaluated for impaired                                                                              | 49,573         | 48,812    |   |
| Total                                                                                                                            | 4,218,810      | 4,128,588 | 4,204,704 |
| Ending balance: individually evaluated for impaired                                                                              | 71,421         | 77,490    |   |
| Ending balance: collectively evaluated for impaired                                                                              | 4,147,389      | 4,051,098 |   |
| Commercial, financial, agricultural and other [Member]                                                                           |           |      |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | 19,852         | 18,200    |   |
| Charge-offs                                                                                                                      | (538)          | (1,914)   |   |
| Recoveries                                                                                                                       | 128            | 238       |   |
| Provision                                                                                                                        | 833            | 1,619     |   |
| Ending Balance                                                                                                                   | 20,275         | 18,143    |   |
| Ending balance: individually evaluated for impaired                                                                              | 9,846          | 7,555     |   |
| Ending balance: collectively evaluated for impaired                                                                              | 10,429         | 10,588    |   |
| Total                                                                                                                            | 1,057,663      | 1,051,209 |   |
| Ending balance: individually evaluated for impaired                                                                              | 33,322         | 31,599    |   |
| Ending balance: collectively evaluated for impaired                                                                              | 1,024,341      | 1,019,610 |   |
| Real estate construction [Member]                                                                                                |           |      |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | 8,928          | 6,756     |   |
| Charge-offs                                                                                                                      | (84)           | (190)     |   |
| Recoveries                                                                                                                       | 12             | 56        |   |
| Provision                                                                                                                        | (1,123)        | (195)     |   |
| Ending Balance                                                                                                                   | 7,733          | 6,427     |   |
| Ending balance: individually evaluated for impaired                                                                              | 503            | 2,891     |   |
| Ending balance: collectively evaluated for impaired                                                                              | 7,230          | 3,536     |   |
| Total                                                                                                                            | 70,461         | 64,158    |   |
| Ending balance: individually evaluated for impaired                                                                              | 8,917          | 13,800    |   |
| Ending balance: collectively evaluated for impaired                                                                              | 61,544         | 50,358    |   |
| Residential real estate [Member]                                                                                                 |           |      |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | 5,908          | 8,237     |   |
| Charge-offs                                                                                                                      | (322)          | (1,712)   |   |
| Recoveries                                                                                                                       | 723            | 133       |   |
| Provision                                                                                                                        | (560)          | 44        |   |
| Ending Balance                                                                                                                   | 5,749          | 6,702     |   |
| Ending balance: individually evaluated for impaired                                                                              | 1,091          | 553       |   |
| Ending balance: collectively evaluated for impaired                                                                              | 4,658          | 6,149     |   |
| Total                                                                                                                            | 1,255,515      | 1,174,572 |   |
| Ending balance: individually evaluated for impaired                                                                              | 7,813          | 3,217     |   |
| Ending balance: collectively evaluated for impaired                                                                              | 1,247,702      | 1,171,355 |   |
| Commercial real estate portfolio segment [Member]                                                                                |           |      |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | 22,441         | 18,961    |   |
| Charge-offs                                                                                                                      | (8,544)        | (235)     |   |
| Recoveries                                                                                                                       | 97             | 158       |   |
| Provision                                                                                                                        | 4,476          | 487       |   |
| Ending Balance                                                                                                                   | 18,470         | 19,371    |   |
| Ending balance: individually evaluated for impaired                                                                              | 1,249          | 921       |   |
| Ending balance: collectively evaluated for impaired                                                                              | 17,221         | 18,450    |   |
| Total                                                                                                                            | 1,243,676      | 1,264,060 |   |
| Ending balance: individually evaluated for impaired                                                                              | 21,369         | 28,874    |   |
| Ending balance: collectively evaluated for impaired                                                                              | 1,222,307      | 1,235,186 |   |
| Loans to individuals [Member]                                                                                                    |           |      |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | 4,132          | 4,244     |   |
| Charge-offs                                                                                                                      | (988)          | (941)     |   |
| Recoveries                                                                                                                       | 94             | 118       |   |
| Provision                                                                                                                        | 901            | 831       |   |
| Ending Balance                                                                                                                   | 4,139          | 4,252     |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 0         |   |
| Ending balance: collectively evaluated for impaired                                                                              | 4,139          | 4,252     |   |
| Total                                                                                                                            | 591,495        | 574,589   |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 0         |   |
| Ending balance: collectively evaluated for impaired                                                                              | 591,495        | 574,589   |   |
| Unallocated [Member]                                                                                                             |           |      |   |
| **Allowance for Doubtful Accounts Receivable [Roll Forward]**                                                                    |           |      |   |
| Beginning Balance                                                                                                                | 5,926          | 4,836     |   |
| Charge-offs                                                                                                                      | 0              | 0         |   |
| Recoveries                                                                                                                       | 0              | 0         |   |
| Provision                                                                                                                        | (30)           | 1,001     |   |
| Ending Balance                                                                                                                   | 5,896          | 5,837     |   |
| Ending balance: individually evaluated for impaired                                                                              | 0              | 0         |   |
| Ending balance: collectively evaluated for impaired                                                                              | $ 5,896        | $ 5,837   |   |

| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Provision for loan lease income (loss).
+ References
No definition available.
+ Details**Name:**fcf_ProvisionForLoanLeaseIncomeLoss**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                     |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AllowanceForDoubtfulAccountsReceivableRollforward**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
A valuation allowance for financing receivables that are expected to be uncollectible.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLosses**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of recovery of financing receivables doubtful of collection that were previously charged off.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of direct write-downs of financing receivables charged against the allowance.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were collectively evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The balance of financing receivables that were individually evaluated for impairment.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953423-111524
+ Details**Name:**us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher OTS -Name Federal Regulation (FR) -Number Title 12 -Section 563c.102 -Paragraph 8 -Subparagraph e -Chapter V -Subsection I -LegacyDoc This is a non-GAAP reference that was included in the 2009 taxonomy.  It will be removed from future versions of this taxonomy.Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-DEP -Paragraph 116 -IssueDate 2006-05-01 -Chapter 8 -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.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=7512638&loc=d3e5074-111524Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6375948&loc=d3e4428-111522Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph e -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_LoansAndLeasesReceivableNetOfDeferredIncome**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |