| **Commitments and Contingent Liabilities - Additional Information (Detail) (USD $)** | 6 Months Ended |           |
|--------------------------------------------------------------------------------------|----------------|-----------|
||
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Notional amount of financial standby letters of credit                               | $ 286,000      |      |
| Notional amount of performance standby letters of credit                             | 500,000        |      |
| Notional amount of commercial letters of credit                                      | 0              |      |
| Financial instrument of credit risk                                                  | 100,000        | 100,000   |
| Number of new complaints                                                             | 3              |      |
| Loss Contingency, Number of Plaintiffs                                               | 36             |      |
| Unused commitments and letters of credit [Member]                                    |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Unfunded commitment liability                                                        | 3,100,000      | 3,200,000 |
| Other Matters [Member]                                                               |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Contingency reserve                                                                  | 100,000        | 800,000   |
| Loss Contingency Paid                                                                | 400,000        |      |
| Accrued Loss Contingency Relieved                                                    | 300,000        |      |
| Other Matters [Member] | Minimum [Member]                                            |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Number of customers affected (in customers)                                          | 700            |      |
| Other Matters [Member] | Maximum [Member]                                            |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Number of customers affected (in customers)                                          | 900            |      |
| Pending Litigation [Member]                                                          |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Percentage Of Interest Rate On Deposits                                              | 3.50%          |      |
| Number of Former Plaintiffs                                                          | 34             |      |
| Specified Average Age of Plaintiffs                                                  | 62             |      |
| Jarrett [Member] | Pending Litigation [Member]                                       |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Loss Contingency, Number of Plaintiffs                                               | 8              |      |
| Young [Member] | Pending Litigation [Member]                                         |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Loss Contingency, Number of Plaintiffs                                               | 12             |      |
| Fisanik [Member] | Pending Litigation [Member]                                       |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Loss Contingency, Number of Plaintiffs                                               | 14             |      |
| Ira Market Rate For Savings Account [Member] | Pending Litigation [Member]           |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Percentage Of Interest Rate On Deposits                                              | 8.00%          |      |
| Market Rate Savings Balance                                                          | 4,000,000      |      |
| Original Plaintiff [Member]                                                          |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Loss Contingency, Damages Sought, Value                                              | 500,000        |      |
| Original Plaintiff's Adult Children [Member]                                         |           |      |
| **Loss Contingencies [Line Items]**                                                  |           |      |
| Loss Contingency, Damages Sought, Value                                              | $ 800,000      |      |

| X                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Accrued Loss Contingency Relieved
+ References
No definition available.
+ Details**Name:**fcf_AccruedLossContingencyRelieved**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest Rate On Deposits
+ References
No definition available.
+ Details**Name:**fcf_InterestRateOnDeposits**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loss Contingency Paid
+ References
No definition available.
+ Details**Name:**fcf_LossContingencyPaid**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Market Rate Savings Balance
+ References
No definition available.
+ Details**Name:**fcf_MarketRateSavingsBalance**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Notional amount of commercial letters of credit.
+ References
No definition available.
+ Details**Name:**fcf_NotionalAmountOfCommercialLettersOfCredit**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Notional Amount Of Financial Standby Letters Of Credit
+ References
No definition available.
+ Details**Name:**fcf_NotionalAmountOfFinancialStandbyLettersOfCredit**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Notional amount of performance standby letters of credit.
+ References
No definition available.
+ Details**Name:**fcf_NotionalAmountOfPerformanceStandbyLettersOfCredit**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**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
Number of Former Plaintiffs
+ References
No definition available.
+ Details**Name:**fcf_NumberofFormerPlaintiffs**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of new complaints
+ References
No definition available.
+ Details**Name:**fcf_Numberofnewcomplaints**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reserve for credit risk of off-balance sheet activities
+ References
No definition available.
+ Details**Name:**fcf_ReserveForOffBalanceSheetActivities**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Specified Average Age of Plaintiffs
+ References
No definition available.
+ Details**Name:**fcf_SpecifiedAverageAgeofPlaintiffs**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 25 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 7Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 17 -Article 9
+ Details**Name:**us-gaap_FairValueConcentrationOfRiskCommitments**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
Amount of loss contingency liability.
+ 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=25496072&loc=d3e14326-108349
+ Details**Name:**us-gaap_LossContingencyAccrualAtCarryingValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The value (monetary amount) of the award the plaintiff seeks in the legal matter.
+ 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=25496072&loc=d3e14326-108349Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349
+ Details**Name:**us-gaap_LossContingencyDamagesSoughtValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| 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 |