| **Fair Values of Assets and Liabilities - Schedule of Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                    |                                                                                                                                                                                                                                          |   |
| Balance, beginning of period                                                                                                                                                                         | $ 30,419us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue                                                                                                                                              | $ 24,943us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
| Total gains or losses included in earnings                                                                                                                                                           | 105us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1                                                                                                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 |
| Total gains or losses included in other comprehensive income                                                                                                                                         | 2,620us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss                                                                                                        | 4,743us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss |
| Purchases                                                                                                                                                                                            | 500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases                                                                                                                                               | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases |
| Issuances                                                                                                                                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues                                                                                                                                                    | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues |
| Sales                                                                                                                                                                                                | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales                                                                                                                                                     | (716)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales |
| Settlements                                                                                                                                                                                          | (590)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements                                                                                                                                           | (390)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3                                                                                                                                       | 716us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 |
| Balance, end of period                                                                                                                                                                               | 33,054us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue                                                                                                                                                | 29,296us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
| Pooled Trust Preferred Collateralized Debt Obligations [Member]                                                                                                                                      |                                                                                                                                                                                                                                          |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                    |                                                                                                                                                                                                                                          |   |
| Balance, beginning of period                                                                                                                                                                         | 28,999us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                         | 23,523us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Total gains or losses included in earnings                                                                                                                                                           | 105us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                      | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Total gains or losses included in other comprehensive income                                                                                                                                         | 2,620us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember | 4,743us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Purchases                                                                                                                                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Issuances                                                                                                                                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                             | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Sales                                                                                                                                                                                                | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                              | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Settlements                                                                                                                                                                                          | (590)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                    | (390)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Balance, end of period                                                                                                                                                                               | 31,134us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember                                         | 27,876us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
| Equity Securities [Member]                                                                                                                                                                           |                                                                                                                                                                                                                                          |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                    |                                                                                                                                                                                                                                          |   |
| Balance, beginning of period                                                                                                                                                                         | 1,420us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                              | 1,420us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Total gains or losses included in earnings                                                                                                                                                           | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Total gains or losses included in other comprehensive income                                                                                                                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                         | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Purchases                                                                                                                                                                                            | 500us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Issuances                                                                                                                                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                                 | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Sales                                                                                                                                                                                                | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                                  | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Settlements                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                            | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                                                                                       | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                    | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Balance, end of period                                                                                                                                                                               | 1,920us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember                                                                              | 1,420us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= us-gaap_EquitySecuritiesMember |
| Loans Held For Sale [Member]                                                                                                                                                                         |                                                                                                                                                                                                                                          |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]**                                                                                    |                                                                                                                                                                                                                                          |   |
| Balance, beginning of period                                                                                                                                                                         |                                                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Total gains or losses included in earnings                                                                                                                                                           |                                                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Total gains or losses included in other comprehensive income                                                                                                                                         |                                                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Purchases                                                                                                                                                                                            |                                                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Issuances                                                                                                                                                                                            |                                                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Sales                                                                                                                                                                                                |                                                                                                                                                                                                                                          | (716)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Settlements                                                                                                                                                                                          |                                                                                                                                                                                                                                          | 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                                                                                       |                                                                                                                                                                                                                                          | 716us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |
| Balance, end of period                                                                                                                                                                               |                                                                                                                                                                                                                                          | $ 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue/ us-gaap_FairValueByAssetClassAxis= fcf_LoansHeldForSaleMember |

| X                                                                                                                                                                                                                                |
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
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