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=28360136&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=27724398&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=6871852&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=27724398&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=36462937&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

The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(h))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Paragraph 13
-Subparagraph h
-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Paragraph 3
-Article 7

+ Details

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

- 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 8
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563

+ Details

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

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=fcf_OtherGovernmentSponsoredEnterprisesMember
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_MmCommIxMember
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:

Impairment of Investment Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) Security Bank	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	$ 891,111,000	$ 1,317,318,000
Estimated Fair Value of Available for Sale	$ 886,560,000	1,309,819,000
Number of banks and other financial institutions comprising the security \| Bank	275
Number Of Pooled Securities with No Senior Class \| Security	3
Number of Securities with No Excess Subordination \| Security	4
Number of Securities with Excess Subordination \| Security	5
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	$ 218,000	0	$ 13,356,000
Net securities (losses) gains	(153,000)	550,000	(1,158,000)
Other investments	$ 62,952,000	44,545,000
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	10.00%
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	20.00%
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	84.00%
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	133.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	40.00%
Corporate/Mortgage-Backed Securities, Amortized cost	$ 42,239,000	41,926,000
Estimated Fair Value of Available for Sale	35,658,000	28,999,000
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	59.00%
Corporate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	$ 1,897,000	6,682,000
Estimated Fair Value of Available for Sale	$ 2,319,000	7,255,000
US States and Political Subdivisions Debt Securities [Member] \| Other Government - Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	1.00%
Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number of securities in an unrealized loss positions \| Security	80
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	$ 2,170,000	1,420,000
Estimated Fair Value of Available for Sale	2,170,000	$ 1,420,000
Mm Comm Ix [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other than-temporary impairment charge	13,400,000
Corporate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Corporate/Mortgage-Backed Securities, Amortized cost	6,600,000
Net securities (losses) gains	$ 1,300,000
Trust preferred collateralized debt obligations [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of Current Performing Collateral	0.00%