Asset Size Of Financial Institution

No definition available.

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

- Definition

Coupon Interest Rate

+ References

No definition available.

+ Details

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

- Definition

Estimate Of Future Cash Flows Percentage In Year One and Two

+ References

No definition available.

+ Details

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

- Definition

Estimate Of Future Cash Flows Prepayment Percentage After Year Two

+ References

No definition available.

+ Details

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

- 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 Input Coupon Rates

+ References

No definition available.

+ Details

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

- 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

Current performing collateral related to the security.

+ References

No definition available.

+ Details

Name:	fcf_PercentageOfCurrentPerformingCollateral
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

Percentage Of Total Unrealized Losses

+ References

No definition available.

+ Details

Name:	fcf_PercentageOfTotalUnrealizedLosses
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=65884222&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=73527792&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=66010992&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

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ 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 (b)(3)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
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=6478792&loc=d3e63345-112809

+ Details

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

- Definition

This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=SL6284393-111563

+ Details

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

- Definition

The excess of the (1) fair value of consideration transferred to the holders of a security in excess of (2) the carrying amount of the security reported on the registrant's balance sheet, which will be deducted from net earnings to derive net earnings available to common shareholders. This amount is generally an adjustment considered in the computation of earnings per share.

+ References

No definition available.

+ Details

Name:	us-gaap_RedemptionPremium
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=fcf_OverFifteenMillionMember
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_FairValueByLiabilityClassAxis=us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember
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_FairValueByLiabilityClassAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DebtSecuritiesMember
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:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fcf_CorporateSecuritiesMember
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_FinancialInstrumentAxis=fcf_MezzanineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Impairment of Investment Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Security Bank	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	$ 790,082,000	$ 891,111,000
Estimated Fair Value of Available for Sale	$ 778,612,000	886,560,000
Number of banks and other financial institutions comprising the security \| Bank	259
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	6
Collateral issued by financial institutions	$ 15,000,000,000
Coupon interest rate	7.00%
Prepayment rate	5.00%
Estimate of future cash flows prepayment percentage in year three	40.00%
Estimate of future cash flows prepayment percentage thereafter	2.00%
Fair Value Input Coupon Rates	10.00%
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%
Other than-temporary impairment charge	$ 6,671,000	218,000	$ 0
Other investments	$ 36,498,000	62,952,000
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%
Collateral issued by financial institutions	$ 15,000,000,000
Probabilities for performing collateral range	0.33%
Excess percent value of future cash flows over current book value	19.00%
Over Fifteen Million [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Prepayment rate	100.00%
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	80.00%
Collateral issued by financial institutions	$ 2,000,000,000
Probabilities for performing collateral range	75.00%
Excess percent value of future cash flows over current book value	119.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	32.00%
Corporate/Mortgage-Backed Securities, Amortized cost	$ 39,989,000	42,239,000
Estimated Fair Value of Available for Sale	33,292,000	35,658,000
Other than-temporary impairment charge	$ 0	0	$ 0
Obligations of U.S. Government-Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	62.00%
Corporate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	$ 5,903,000	1,897,000
Estimated Fair Value of Available for Sale	$ 6,319,000	2,319,000
Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of securities in an unrealized loss positions \| Security	122
Estimated Fair Value of Available for Sale	$ 776,942,000	884,390,000
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of securities in an unrealized loss positions \| Security	0
Corporate/Mortgage-Backed Securities, Amortized cost	$ 1,670,000	2,170,000
Estimated Fair Value of Available for Sale	1,670,000	2,170,000
Corporate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Estimated Fair Value of Available for Sale	$ 6,319,000	$ 2,319,000
Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of Current Performing Collateral	0.00%
Mezzanine [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	$ 39,989,000
Estimated Fair Value of Available for Sale	33,292,000
Mezzanine [Member] \| Pre TSL VII [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	3,100,000
Net securities gains (losses)	$ 600,000