Available-for-sale, securities in unrealized loss, percentage.

No definition available.

Name:	fcf_AvailableForSaleSecuritiesInUnrealizedLossPercentage
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Excess present value of future cash flows over our current book value.

+ References

No definition available.

+ Details

Name:	fcf_ExcessPresentValueOfFutureCashFlowsOverOurCurrentBookValue
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Excess subordination as a percentage of current performing collateral.

+ References

No definition available.

+ Details

Name:	fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs, Collateral

+ References

No definition available.

+ Details

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

- Definition

Fair Value Inputs, Coupon Rate

+ References

No definition available.

+ Details

Name:	fcf_FairValueInputsCouponRate
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs , Prepayment Rate After Year Two

+ References

No definition available.

+ Details

Name:	fcf_FairValueInputsPrepaymentRateAfterYearTwo
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair Value Inputs , Prepayment Rate Year One and Two

+ References

No definition available.

+ Details

Name:	fcf_FairValueInputsPrepaymentRateYearOneAndTwo
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss

+ References

No definition available.

+ Details

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

- Definition

Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total

+ References

No definition available.

+ Details

Name:	fcf_InvestmentSecuritiesinUnrealizedLossPositionQualitativeDisclosureNumberofPositionsTotal
Namespace Prefix:	fcf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of banks and other financial institutions comprising the security.

+ References

No definition available.

+ Details

Name:	fcf_NumberOfBanksAndOtherFinancialInstitutionsComprisingSecurity
Namespace Prefix:	fcf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number Of Pooled Securities with No Senior Class

+ References

No definition available.

+ Details

Name:	fcf_NumberOfPooledSecuritiesWithNoSeniorClass
Namespace Prefix:	fcf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of Securities with Excess Subordination

+ References

No definition available.

+ Details

Name:	fcf_NumberOfSecuritiesWithExcessSubordination
Namespace Prefix:	fcf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Securities with No Excess Subordination

+ References

No definition available.

+ Details

Name:	fcf_NumberOfSecuritiesWithNoExcessSubordination
Namespace Prefix:	fcf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percent of Unrealized Losses

+ References

No definition available.

+ Details

Name:	fcf_PercentOfUnrealizedLosses
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of principal amount in pooled security securities.

+ References

No definition available.

+ Details

Name:	fcf_PercentageOfPrincipalAmountInPooledSecuritySecurities
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Probabilities for performing collateral range.

+ References

No definition available.

+ Details

Name:	fcf_ProbabilitiesForPerformingCollateralRange
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Probability of default.

+ References

No definition available.

+ Details

Name:	fcf_ProbabilityOfDefault
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Probability percentage assigned to default bank.

+ References

No definition available.

+ Details

Name:	fcf_ProbabilityPercentageAssignedToDefaultBank
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Probability percentage assigned to default bank subject to market indicators.

+ References

No definition available.

+ Details

Name:	fcf_ProbabilityPercentageAssignedToDefaultBankSubjectToMarketIndicators
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Projected recovery rate.

+ References

No definition available.

+ Details

Name:	fcf_ProjectedRecoveryRate
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Subordinated tranches range.

+ References

No definition available.

+ Details

Name:	fcf_SubordinatedTranchesRange
Namespace Prefix:	fcf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ 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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

+ Details

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

- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ 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 (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsPrepaymentRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 325
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809

+ Details

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

- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fcf_TrustPreferredCollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fcf_PooledTrustPreferredSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_EstimateOfFutureCashFlowAxis=fcf_AssetSizeAndCouponRate1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_EstimateOfFutureCashFlowAxis=fcf_AssetSizeAndCouponRate2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_EstimateOfFutureCashFlowAxis=fcf_AssetSizeAndCouponRate3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_EstimateOfFutureCashFlowAxis=fcf_AssetSizeAndCouponRate4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcf_EstimateOfFutureCashFlowAxis=fcf_AssetSizeAndCouponRate5Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Impairment of Investment Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) Bank Security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other than temporary impairment charges recognized	$ 0	$ 0
Amortized Cost	829,188,000		$ 740,846,000
Securities available for sale, at fair value	812,877,000		731,358,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 31,725,000		18,534,000
Number of banks and other financial institutions comprising the security \| Bank	164
Number Of Pooled Securities with No Senior Class \| Security	2
Number of securities with no excess subordination \| Security	2
Number of Securities with Excess Subordination \| Security	4
Coupon rate	7.00%
Probability of default	100.00%
Projected recovery rate	0.00%
Probability percentage assigned to default bank subject to market indicators	10.00%
Probability percentage assigned to default bank	100.00%
Federal Home Loan Bank Stock	$ 24,400,000		29,837,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total	155
Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Subordinated tranches range	7.00%
Total principal amount of the respective securities	5.00%
Excess subordination as a percentage of current performing collateral	2.00%
Probabilities for performing collateral range	0.33%
Excess present value of future cash flows over our current book value	19.00%
Maximum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Subordinated tranches range	35.00%
Excess subordination as a percentage of current performing collateral	114.00%
Probabilities for performing collateral range	50.00%
Excess present value of future cash flows over our current book value	101.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	2.00%
Securities available for sale, at fair value	$ 14,132,000		23,646,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	722,000		4,379,000
Trust preferred collateralized debt obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other than temporary impairment charges recognized	$ 0	0
Percentage of Current Performing Collateral	0.00%
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale, at fair value	$ 21,226,000		16,493,000
Pooled Trust Preferred Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other than temporary impairment charges recognized	0	0
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other than temporary impairment charges recognized	0	$ 0
Continuous Unrealized Loss Position, Fair Value	0		0
Asset Size and Coupon Rate 1 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral issued by financial institutions	$ 15,000,000,000
Coupon rate	7.00%
Prepayment rate	100.00%
Asset Size and Coupon Rate 2 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Coupon rate	7.00%
Fair Value Inputs , Prepayment Rate Year One and Two	40.00%
Fair Value Inputs , Prepayment Rate After Year Two	2.00%
Asset Size and Coupon Rate 3 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Prepayment rate	5.00%
Asset Size and Coupon Rate 3 [Member] \| Minimum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral issued by financial institutions	$ 2,000,000,000
Asset Size and Coupon Rate 3 [Member] \| Maximum [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral issued by financial institutions	15,000,000,000
Asset Size and Coupon Rate 4 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Collateral issued by financial institutions	$ 2,000,000,000
Coupon rate	10.00%
Prepayment rate	5.00%
Asset Size and Coupon Rate 5 [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Prepayment rate	0.00%
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percent of Unrealized Losses	83.00%
Reported Value Measurement [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Securities available for sale, at fair value	$ 812,877,000		$ 731,358,000
Federal Home Loan Bank Stock	$ 24,400,000