| **Fair Values of Assets and Liabilities - Schedule of Assets Measured on Non-Recurring Basis (Detail) (USD $)In Thousands, unless otherwise specified** | Jun. 30, 2014 | Dec. 31, 2013 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| Fair Value, Measurements, Nonrecurring [Member]                                                                                                         |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Impaired loans                                                                                                                                          | $ 40,215      | $ 50,559      |
| Other real estate owned                                                                                                                                 | 8,360         | 12,924        |
| Total Assets                                                                                                                                            | 48,575        | 63,483        |
| Level 1 [Member]                                                                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Total Assets                                                                                                                                            | 0             | 0             |
| Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Impaired loans                                                                                                                                          | 0             | 0             |
| Other real estate owned                                                                                                                                 | 0             | 0             |
| Total Assets                                                                                                                                            | 0             | 0             |
| Level 2 [Member]                                                                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Total Assets                                                                                                                                            | 1,373,218     | 1,343,224     |
| Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member]                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Impaired loans                                                                                                                                          | 27,342        | 36,903        |
| Other real estate owned                                                                                                                                 | 8,221         | 12,752        |
| Total Assets                                                                                                                                            | 35,563        | 49,655        |
| Level 3 [Member]                                                                                                                                        |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Total Assets                                                                                                                                            | 29,574        | 24,943        |
| Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]                                                                                      |          |          |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                        |          |          |
| Impaired loans                                                                                                                                          | 12,873        | 13,656        |
| Other real estate owned                                                                                                                                 | 139           | 172           |
| Total Assets                                                                                                                                            | $ 13,012      | $ 13,828      |

| X                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impaired loans fair value.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other real estate owned assets fair value.
+ References
No definition available.
+ Details**Name:**fcf_OtherRealEstateOwnedAssetsFairValue**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
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| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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