Derivative Instruments Designated As Hedging Instruments Interest Expense

No definition available.

Name:	fcf_DerivativeInstrumentsDesignatedAsHedgingInstrumentsInterestExpense
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Derivative Liability, Cost Basis, Gross Asset

+ References

No definition available.

+ Details

Name:	fcf_DerivativeLiabilityCostBasisGrossAsset
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Derivative Liability, Notional Amount, Gross Asset

+ References

No definition available.

+ Details

Name:	fcf_DerivativeLiabilityNotionalAmountGrossAsset
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

[Line Items] for Risk participation Agreements [Table]

+ References

No definition available.

+ Details

Name:	fcf_RiskparticipationAgreementsLineItems
Namespace Prefix:	fcf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of the assets less the liabilities of a derivative or group of derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-10

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-SubTopic 10
-Topic 825
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482881/825-10-55-10

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-7A

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 21
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-21

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 20
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483444/210-20-55-20

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483466/210-20-50-3

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5C
-Subparagraph (SX 210.12-13C(Column F))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5C

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5B
-Subparagraph (SX 210.12-13B(Column E))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5B

+ Details

Name:	us-gaap_DerivativeLiabilityFairValueGrossAsset
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480401/815-10-55-181

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The underlying refers to the price on the security, commodity, index, or other variable specified by the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeUnderlying
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of credit risk derivative instruments held.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_NumberOfCreditRiskDerivativesHeld
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	fcf_RiskParticipationAgreementsAxis=fcf_RiskParticipationAgreementsPurchasedDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_RiskParticipationAgreementsAxis=fcf_RiskParticipationAgreementsSoldDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForwardContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fcf_InterestRateMacroSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_MaturityDateAxis=fcf_A512026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_MaturityDateAxis=fcf_A10152026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_MaturityDateAxis=fcf_A2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_MaturityDateAxis=fcf_May2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_MaturityDateAxis=fcf_August2025Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fcf_InterestRateSwapMemberDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_ContractMaturityAxis=fcf_A2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_ContractMaturityAxis=fcf_A2026Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivatives - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2026 USD ($) Derivatives	Mar. 31, 2025 USD ($)	Dec. 31, 2025 USD ($)
Risk participation Agreements [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (6)	$ (153)
Risk Participation Agreements Purchased [Domain]
Risk participation Agreements [Line Items]
Number of interest rate swaps \| Derivatives	22
Risk Participation Agreements Sold [Domain]
Risk participation Agreements [Line Items]
Number of interest rate swaps \| Derivatives	20
Not Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Fair Value, Net	$ 191		$ 185
Interest Rate Swap [Member]
Risk participation Agreements [Line Items]
Derivative Instruments Designated As Hedging Instruments Interest Expense	243	311
Derivative, Gain (Loss) on Derivative, Net	(1,100)	(3,700)
Interest Rate Swap [Member] \| Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Fair Value, Net	(401)		766
Derivative, Notional Amount	215,000		215,000
Derivative, Gain (Loss) on Derivative, Net	(200)
Interest Rate Swap [Member] \| Not Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	992,194		1,000,541
Forward Contracts [Member]
Risk participation Agreements [Line Items]
Derivative Liability, Cost Basis, Gross Asset	7,900		19,500
Derivative Liability, Fair Value, Gross Asset	9,300		21,500
Derivative Liability, Notional Amount, Gross Asset	61,200		48,100
Forward Contracts [Member] \| Not Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Fair Value, Net	(268)		133
Derivative, Notional Amount	54,000		49,000
Derivative, Gain (Loss) on Derivative, Net	401	$ (669)
Interest Rate Contract [Member] \| Not Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	61,187		$ 48,143
Interest Rate Macro Swap \| Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	$ 175,000
Derivative, Underlying	175.0 million
Interest Rate Macro Swap \| Designated as Hedging Instrument [Member] \| 5/1/2026
Risk participation Agreements [Line Items]
Derivative, Notional Amount	$ 150,000
Interest Rate Macro Swap \| Designated as Hedging Instrument [Member] \| 10/15/2026
Risk participation Agreements [Line Items]
Derivative, Notional Amount	25,000
Interest Rate Macro Swap \| Designated as Hedging Instrument [Member] \| 2024 [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	75,000
Interest Rate Macro Swap \| Designated as Hedging Instrument [Member] \| 2025
Risk participation Agreements [Line Items]
Derivative, Notional Amount	150,000
Interest Rate Macro Swap \| Designated as Hedging Instrument [Member] \| August 2025
Risk participation Agreements [Line Items]
Derivative, Notional Amount	25,000
Interest Rate Swap Member - Debt \| Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	40,000
2024 [Member] \| Interest Rate Swap [Member] \| Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	30,000
2026 [Member] \| Interest Rate Swap [Member] \| Designated as Hedging Instrument [Member]
Risk participation Agreements [Line Items]
Derivative, Notional Amount	$ 40,000