Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022
Cash paid during the period for:
Interest	$ 52,038	$ 5,946
Income Taxes Paid, Net	16,924	16,621
Non-cash investing and financing activities:
Loans Transferred to Other Real Estate Owned and Repossessions	1,443	1,313
Transfer of Loans Held-for-sale to Portfolio Loans	(519)	0
Fair value of loans transferred from held to maturity to held for sale	60,611	31,519
Gross decrease in market value adjustment to securities available for sale	(3,761)	(89,046)
Gross increase (decrease) in market value adjustment to derivatives	2,225	(22,671)
Stock Issued During Period, Value, Treasury Stock Reissued	1,966	1,947
Noncash or Part Noncash Acquisition, Value of Assets Acquired	66,477	0
Proceeds from reissuance of treasury stock	0	625
Other Significant Noncash Transaction, Value of Consideration Given	$ 3,350	$ 0

Cash paid for interest and income taxes during the period.

No definition available.

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

- Definition

Gross increase in market value adjustment to securities available for sale.

+ References

No definition available.

+ Details

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

- Definition

Gross Increase in Market Value Adjustment to Derivatives

+ References

No definition available.

+ Details

Name:	fcf_GrossIncreaseinMarketValueAdjustmenttoDerivatives
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Loans Transferred to Other Real Estate Owned and Repossessions

+ References

No definition available.

+ Details

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

- Definition

Unsettled Payments for the Acquisition of Treasury Stock

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-2

+ Details

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

- Definition

Amount of cash paid for interest, excluding capitalized interest, classified as operating activity. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 17
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-17

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 25
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-25

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-2

+ Details

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

- Definition

The value of an asset or business acquired in a noncash (or part noncash) acquisition. Noncash is defined as information about all investing and financing activities of an enterprise during a period that affect recognized assets or liabilities but that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-4

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-3

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482913/230-10-50-5

+ Details

Name:	us-gaap_NoncashOrPartNoncashAcquisitionValueOfAssetsAcquired1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The value of the noncash (or part noncash) consideration given (for example, liability, equity) in a transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of a transaction not resulting in cash receipts or cash payments in the period.

+ References

+ Details

Name:	us-gaap_OtherSignificantNoncashTransactionValueOfConsiderationGiven1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The fair value of stock issued in noncash financing activities.

+ References

+ Details

Name:	us-gaap_StockIssued1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Value of loans held-for-sale transferred to loans held in the entity's portfolio in noncash transactions.

+ References

+ Details

Name:	us-gaap_TransferOfLoansHeldForSaleToPortfolioLoans1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Value of loans and leases held in the entity's portfolio transferred to loans and leases held-for-sale in noncash transactions.

+ References

+ Details

Name:	us-gaap_TransferOfPortfolioLoansAndLeasesToHeldForSale1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration