| **Goodwill and Other Amortizing Intangible Assets - Summarized Other Intangible Assets (Detail) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                                 | Dec. 31, 2013                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Finite-Lived Intangible Assets [Line Items]**                                                                                                   |                                                                                                                                                          |                                                                                                                                                          |
| Gross Intangible Assets                                                                                                                           | $ 23,454us-gaap_FiniteLivedIntangibleAssetsGross                                                                                                              | $ 22,470us-gaap_FiniteLivedIntangibleAssetsGross                                                                                                              |
| Accumulated Amortization                                                                                                                          | (21,789)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization                                                                                            | (21,159)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization                                                                                            |
| Net Intangible Assets                                                                                                                             | 1,665us-gaap_FiniteLivedIntangibleAssetsNet                                                                                                                   | 1,311us-gaap_FiniteLivedIntangibleAssetsNet                                                                                                                   |
| Customer Deposit Intangibles [Member]                                                                                                             |                                                                                                                                                          |                                                                                                                                                          |
| **Finite-Lived Intangible Assets [Line Items]**                                                                                                   |                                                                                                                                                          |                                                                                                                                                          |
| Gross Intangible Assets                                                                                                                           | 22,470us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= fcf_CustomerDepositIntangiblesMember                     | 22,470us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= fcf_CustomerDepositIntangiblesMember                     |
| Accumulated Amortization                                                                                                                          | (21,773)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= fcf_CustomerDepositIntangiblesMember | (21,159)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= fcf_CustomerDepositIntangiblesMember |
| Net Intangible Assets                                                                                                                             | 697us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= fcf_CustomerDepositIntangiblesMember                          | 1,311us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= fcf_CustomerDepositIntangiblesMember                        |
| Customer Lists [Member]                                                                                                                           |                                                                                                                                                          |                                                                                                                                                          |
| **Finite-Lived Intangible Assets [Line Items]**                                                                                                   |                                                                                                                                                          |                                                                                                                                                          |
| Gross Intangible Assets                                                                                                                           | 984us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerListsMember                                 | 0us-gaap_FiniteLivedIntangibleAssetsGross/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerListsMember                                   |
| Accumulated Amortization                                                                                                                          | (16)us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerListsMember              | 0us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerListsMember                 |
| Net Intangible Assets                                                                                                                             | $ 968us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerListsMember                                 | $ 0us-gaap_FiniteLivedIntangibleAssetsNet/ us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis= us-gaap_CustomerListsMember                                   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
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| X                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275
+ Details**Name:**us-gaap_FiniteLivedIntangibleAssetsNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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