Business Combinations (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2019	Dec. 31, 2019
Business Combinations [Abstract]
Payments to Acquire Businesses, Net of Cash Acquired	$ 329,533
Business Combination, Acquired Receivables, Gross Contractual Amount		$ 101,200
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Deposits Contractual Amount		471,386
Business Combination Acquired Receivables Fair Value Adjustment	(1,200)
BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDepositsGross		471,000
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deposits Fair Value Adjustment		400
Goodwill, Acquired During Period	29,126
Business Combination, Acquisition Related Costs	$ 3,700
Business Combination, Acquired Receivable, Fair Value		$ 99,956

Business Combination Acquired Receivables Fair Value Adjustment

No definition available.

Name:	fcf_BusinessCombinationAcquiredReceivablesFairValueAdjustment
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Deposits Contractual Amount

+ References

No definition available.

+ Details

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Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deposits Fair Value Adjustment

+ References

No definition available.

+ Details

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Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDepositsGross

+ References

No definition available.

+ Details

Name:	fcf_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDepositsGross
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of acquired receivable from business combination, excluding certain loans and debt securities acquired in transfer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=121647850&loc=d3e4845-128472

+ Details

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

- Definition

For receivables acquired in a business combination, excluding certain loans and debt securities acquired in a transfer (as defined), this element represents the gross contractual amounts receivable, by major class of receivable, such as loans, direct finance leases (as defined), and any other class of receivables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=121647850&loc=d3e4845-128472

+ Details

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

- Definition

This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e1043-128460

+ Details

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

- References

No definition available.

+ Details

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- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121556970&loc=d3e13816-109267

+ Details

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

- Definition

The cash outflow associated with the acquisition of a business, net of the cash acquired from the purchase.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3213-108585

+ Details

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