| **Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Net Periodic Cost (Detail) (Net Periodic Cost [Member])** | 12 Months Ended                                                                                                                                                                            |
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| Net Periodic Cost [Member]                                                                                                                |                                                                                                                                                                                       |   |   |
| **Net Periodic Benefit Cost Assumptions [Line Items]**                                                                                    |                                                                                                                                                                                       |   |   |
| Discount rate                                                                                                                             | 4.01%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember | 3.31%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember | 4.22%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember |
| Health care cost trend: Initial                                                                                                           | 6.75%fcf_DefinedBenefitPlanInitialHealthCareCostTrendRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember                                   | 7.00%fcf_DefinedBenefitPlanInitialHealthCareCostTrendRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember | 8.00%fcf_DefinedBenefitPlanInitialHealthCareCostTrendRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember |
| Health care cost trend: Ultimate                                                                                                          | 4.75%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember                              | 4.75%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember | 4.75%us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember |
| Year ultimate reached                                                                                                                     | 2022                                                                                                                                                                                       | 2022 | 2016 |
| Corridor                                                                                                                                  | 10.00%fcf_DefinedBenefitPlanCorridorRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember                                                    | 10.00%fcf_DefinedBenefitPlanCorridorRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember | 10.00%fcf_DefinedBenefitPlanCorridorRate/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis= fcf_PlansNetPeriodicCostMember |
| Recognition period for gains and losses                                                                                                   | 10 years                                                                                                                                                                                   | 11 years | 12 years |

| X                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan Assumptions Used Calculating Net Periodic Benefit Cost Recognized Period
+ References
No definition available.
+ Details**Name:**fcf_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRecognizedPeriod**Namespace Prefix:**fcf_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan Corridor Rate
+ References
No definition available.
+ Details**Name:**fcf_DefinedBenefitPlanCorridorRate**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Defined Benefit Plan Initial Health Care Cost Trend Rate
+ References
No definition available.
+ Details**Name:**fcf_DefinedBenefitPlanInitialHealthCareCostTrendRate**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_NetPeriodicBenefitCostAssumptionsLineItems**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The interest rate used to adjust for the time value of money.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
+ Details**Name:**us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The ultimate trend rate for health care costs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
+ Details**Name:**us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Year the ultimate health care cost trend rate is expected to be reached, in CCYY format.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920
+ Details**Name:**us-gaap_DefinedBenefitPlanYearThatRateReachesUltimateTrendRate**Namespace Prefix:**us-gaap_**Data Type:**xbrli:gYearItemType**Balance Type:**na**Period Type:**instant |