| **Contingent Liabilities - Additional Information (Detail) (USD $)In Millions, unless otherwise specified**                    | 1 Months Ended | 12 Months Ended |
|--------------------------------------------------------------------------------------------------------------------------------|----------------|-----------------|
||
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of plaintiffs                                                                                                           |           | 36              |
| Pending Litigation [Member]                                                                                                    |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of new complaints                                                                                                       | 3              |            |
| Number of former members of McGrogan class                                                                                     | 34             | 34              |
| Specified average age of plaintiffs                                                                                            |           | 62 years        |
| Ira Market Rate For Savings Account [Member] | Pending Litigation [Member]                                                     |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Market rate savings balance                                                                                                    | $ 4            | $ 4             |
| Minimum [Member] | Unasserted Claim [Member]                                                                                   |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of Customer Affected                                                                                                    | 700            | 700             |
| Maximum [Member] | Unasserted Claim [Member]                                                                                   |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of Customer Affected                                                                                                    | 900            | 900             |
| McGrogan Class Action [Member] | Ira Market Rate For Savings Account [Member] | Pending Litigation [Member]                    |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Interest rate on deposits                                                                                                      |           | 3.50%           |
| McGrogan Class Action [Member] | Minimum [Member] | Ira Market Rate For Savings Account [Member] | Pending Litigation [Member] |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Interest rate on deposits                                                                                                      |           | 8.00%           |
| Jarrett [Member] | Pending Litigation [Member]                                                                                 |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of plaintiffs                                                                                                           |           | 8               |
| Young [Member] | Pending Litigation [Member]                                                                                   |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of plaintiffs                                                                                                           |           | 12              |
| Fisanik [Member] | Pending Litigation [Member]                                                                                 |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Number of plaintiffs                                                                                                           |           | 14              |
| Other Liabilities [Member] | Unasserted Claim [Member]                                                                         |           |            |
| **Loss Contingencies [Line Items]**                                                                                            |           |            |
| Loss Contingency Accrual, Carrying Value, Period Increase (Decrease)                                                           |           | $ 0.8           |

| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loss Contingency, Former Members of Class Action Lawsuit
+ References
No definition available.
+ Details**Name:**fcf_LossContingencyFormerMembersofClassActionLawsuit**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of Customer Affected
+ References
No definition available.
+ Details**Name:**fcf_NumberOfCustomerAffected**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of interest rate on deposits.
+ References
No definition available.
+ Details**Name:**fcf_PercentageOfInterestRateOnDeposits**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Specified average age of account holders.
+ References
No definition available.
+ Details**Name:**fcf_SpecifiedAverageAgeOfAccountHolders**Namespace Prefix:**fcf_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of interest-bearing domestic savings deposit liabilities. Saving deposits bear interest and have no stated maturity.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.12) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 9
+ Details**Name:**us-gaap_InterestBearingDomesticDepositSavings**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_LossContingenciesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The increase (decrease) in the period in loss contingency liabilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6952336&loc=d3e14326-108349Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 9, 10, 11, 12 -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_LossContingencyAccrualCarryingValuePeriodIncreaseDecrease**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The total number of new claims filed pertaining to a loss contingency during the period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 9, 10, 11, 12 -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.
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| X                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of plaintiffs that have filed claims pertaining to a loss contingency.
+ References
No definition available.
+ Details**Name:**us-gaap_LossContingencyNumberOfPlaintiffs**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |