| **Incentive Compensation Plan Incentive Compensation Plan - Restricted Stock Awards Estimated to be Granted on a Performance Award (Details)** | 0 Months Ended                                                                                                                                                                                                                             | 1 Months Ended                                                                                                                                                                                                                                                                               | 12 Months Ended                                                                                                                                                                                                                           | 0 Months Ended | 1 Months Ended                                                                                                                                                                                                                       |
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| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                             |                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Target Award Participants Earn                                 | 40.00%fcf_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTargetAwardParticipantsEarn                                                                                                                                 | 40.00%fcf_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTargetAwardParticipantsEarn                                                                                                                                                                                   | 40.00%fcf_ShareBasedCompensationArrangementByShareBasedPaymentAwardPercentageOfTargetAwardParticipantsEarn                                                                                                                                |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Performance Shares Awards As Percentage Of Target                                                                                              | 100.00%fcf_PerformanceSharesAwardsAsPercentageOfTarget                                                                                                                                                                                     | 100.00%fcf_PerformanceSharesAwardsAsPercentageOfTarget                                                                                                                                                                                                                                       | 100.00%fcf_PerformanceSharesAwardsAsPercentageOfTarget                                                                                                                                                                                    |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Percentage Of Target Award Opportunities Maximum In Certain Cases                                                                              | 200.00%fcf_PercentageOfTargetAwardOpportunitiesMaximumInCertainCases                                                                                                                                                                       | 200.00%fcf_PercentageOfTargetAwardOpportunitiesMaximumInCertainCases                                                                                                                                                                                                                         | 200.00%fcf_PercentageOfTargetAwardOpportunitiesMaximumInCertainCases                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Vesting period of restricted stock                                                                                                             | 1 year                                                                                                                                                                                                                                     | 1 year                                                                                                                                                                                                                                                                                       | 1 year                                                                                                                                                                                                                                    | 10 years       |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Performance Based Stock Awards Vesting Period                                                                                                  | 3 years                                                                                                                                                                                                                                    | 3 years                                                                                                                                                                                                                                                                                      | 3 years                                                                                                                                                                                                                                   |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Restricted Stock [Member] | Executive [Member]                                                                                                 |                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                             |                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Common stock issued to executive                                                                                                               | 128,611,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 68,000,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember                                                    | 54,166,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember |           | 3,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 45,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 10,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 100,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 25,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 10,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember | 30,120us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember |
| Restricted Stock [Member] | Executive [Member] | Long Term Incentive Plan [Member]                                                             |                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**                                                             |                                                                                                                                                                                                                                       |                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |
| Common stock issued to executive                                                                                                               |                                                                                                                                                                                                                                       | 34,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod/ us-gaap_AwardTypeAxis= us-gaap_RestrictedStockMember/ us-gaap_PlanNameAxis= fcf_LongTermIncentivePlanMember/ us-gaap_RelatedPartyTransactionsByRelatedPartyAxis= us-gaap_ExecutiveOfficerMember |                                                                                                                                                                                                                                      |           |                                                                                                                                                                                                                                 |   |   |   |   |   |   |

| X                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage Of Target Award Opportunities Maximum In Certain Cases
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No definition available.
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| X                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Performance Based Stock Awards Vesting Period
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Performance Shares Awards As Percentage Of Target
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Target Award Participants Earn
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
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| X                                                                                                                                                                                                      |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of share instruments newly issued under a share-based compensation plan.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
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| X                                                                                                                                           |
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