| **Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Cash paid during the period for:**                                                                                                          |           |   |
| Interest                                                                                                                                      | $ 7,211        | $ 9,668 |
| **Non-cash investing and financing activities:**                                                                                              |           |   |
| ESOP loan reductions                                                                                                                          | 0              | 500 |
| Loans transferred to other real estate owned and repossessed assets                                                                           | 1,207          | 2,561 |
| Loans transferred from held to maturity to available for sale                                                                                 | 16,613         | 0 |
| Loans sold, not settled                                                                                                                       | 2,640          | 0 |
| Gross increase in market value adjustment to securities available for sale                                                                    | $ (2,411)      | $ 1,597 |

| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Employee stock ownership plan loan reductions.
+ References
No definition available.
+ Details**Name:**fcf_EmployeeStockOwnershipPlanLoanReductions**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gross increase in market value adjustment to securities available for sale.
+ References
No definition available.
+ Details**Name:**fcf_GrossIncreaseInMarketValueAdjustmentToSecuritiesAvailableForSale**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans Sold, Not Settled
+ References
No definition available.
+ Details**Name:**fcf_LoansSoldNotSettled**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of loans transferred from the entity's held to maturity loan portfolio to held for sale loan portfolio in noncash transactions.
+ References
No definition available.
+ Details**Name:**fcf_TransferOfLoans**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of other real estate transferred from the entity's loan portfolio in noncash transactions.
+ References
No definition available.
+ Details**Name:**fcf_TransferOfOtherRealEstateOwned**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash paid for interest during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 29 -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 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3536-108585
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