Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment Securities [Line Items]
Amortized Cost	$ 1,033,361	$ 805,515
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	17,822	25,708
Gross Unrealized Losses	(7,925)	0
Securities available for sale, at fair value	1,043,258	831,223
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,067	6,492
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	623	738
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	6,690	7,230
US Government Agency Debt Securities Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	268,890	182,823
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,597	8,357
Gross Unrealized Losses	(586)	0
Securities available for sale, at fair value	272,901	191,180
Residential Mortgage Backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	724,611	481,109
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11,082	14,924
Gross Unrealized Losses	(7,025)	0
Securities available for sale, at fair value	728,668	496,033
US Government-sponsored Enterprises Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,000	100,996
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0	2
Gross Unrealized Losses	(25)	0
Securities available for sale, at fair value	975	100,998
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	9,647	11,154
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	132	243
Gross Unrealized Losses	(214)	0
Securities available for sale, at fair value	9,565	11,397
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	23,146	22,941
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,388	1,444
Gross Unrealized Losses	(75)	0
Securities available for sale, at fair value	$ 24,459	$ 24,385

Investment Securities [Line Items]

No definition available.

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

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

+ Details

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

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

+ Details

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

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=fcf_USGovernmentAgencyDebtSecuritiesResidentialDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=fcf_USGovernmentAgencyDebtSecuritiesCommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: