Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2016 USD ($) Bank	Dec. 31, 2015 USD ($)
Impairment Of Investment Securities [Line Items]
Securities available for sale, at fair value	$ 890,198	$ 886,560
Unrealized Gain (Loss)	11,405	$ 16,477
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Impairment Of Investment Securities [Line Items]
Book Value	42,500
Securities available for sale, at fair value	33,273
Unrealized Gain (Loss)	(9,227)
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL IV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	1,830
Securities available for sale, at fair value	1,291
Unrealized Gain (Loss)	$ (539)
Debt Instrument, Credit Rating	B1/BB
Number of Banks (in banks) \| Bank	6
Deferrals and Defaults as a % of Current Collateral	18.05%
Excess Subordination as a % of Current Performing Collateral	59.28%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL VII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 3,020
Securities available for sale, at fair value	3,272
Unrealized Gain (Loss)	$ 252
Debt Instrument, Credit Rating	Ca/-
Number of Banks (in banks) \| Bank	14
Deferrals and Defaults as a % of Current Collateral	49.68%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL VIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,020
Securities available for sale, at fair value	1,705
Unrealized Gain (Loss)	$ (315)
Debt Instrument, Credit Rating	C/C
Number of Banks (in banks) \| Bank	28
Deferrals and Defaults as a % of Current Collateral	53.00%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL IX [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 2,385
Securities available for sale, at fair value	1,778
Unrealized Gain (Loss)	$ (607)
Debt Instrument, Credit Rating	B1/C
Number of Banks (in banks) \| Bank	38
Deferrals and Defaults as a % of Current Collateral	29.80%
Excess Subordination as a % of Current Performing Collateral	9.82%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL X [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 1,644
Securities available for sale, at fair value	1,779
Unrealized Gain (Loss)	$ 135
Debt Instrument, Credit Rating	Caa1/C
Number of Banks (in banks) \| Bank	43
Deferrals and Defaults as a % of Current Collateral	30.66%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL XII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 5,735
Securities available for sale, at fair value	4,324
Unrealized Gain (Loss)	$ (1,411)
Debt Instrument, Credit Rating	B3/C
Number of Banks (in banks) \| Bank	66
Deferrals and Defaults as a % of Current Collateral	22.03%
Excess Subordination as a % of Current Performing Collateral	0.00%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL XIII [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 12,767
Securities available for sale, at fair value	9,856
Unrealized Gain (Loss)	$ (2,911)
Debt Instrument, Credit Rating	Ba3/C
Number of Banks (in banks) \| Bank	56
Deferrals and Defaults as a % of Current Collateral	12.11%
Excess Subordination as a % of Current Performing Collateral	42.17%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| Pre TSL XIV [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 12,889
Securities available for sale, at fair value	8,969
Unrealized Gain (Loss)	$ (3,920)
Debt Instrument, Credit Rating	B1/CC
Number of Banks (in banks) \| Bank	56
Deferrals and Defaults as a % of Current Collateral	19.72%
Excess Subordination as a % of Current Performing Collateral	49.72%
Pooled Trust Preferred Collateralized Debt Obligations [Member] \| Mezzanine [Member] \| MMCap I [Member]
Impairment Of Investment Securities [Line Items]
Book Value	$ 210
Securities available for sale, at fair value	299
Unrealized Gain (Loss)	$ 89
Debt Instrument, Credit Rating	Ca/C
Number of Banks (in banks) \| Bank	8
Deferrals and Defaults as a % of Current Collateral	58.11%
Excess Subordination as a % of Current Performing Collateral	83.30%

Available for sale securities book value.

No definition available.

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

- Definition

Deferrals and defaults as a percentage of current collateral.

+ References

No definition available.

+ Details

Name:	fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral
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

Impairment Of Investment Securities [Line Items]

+ References

No definition available.

+ Details

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

- Definition

Pooled trust preferred collateralized debt obligations in number of banks.

+ References

No definition available.

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=27724398&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

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=51813488&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=49167202&loc=d3e26610-111562

+ Details

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

- Definition

Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 22
-Article 5

+ Details

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

- Details

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

- Details

Name:	us-gaap_EligibleItemOrGroupForFairValueOptionAxis=fcf_MezzanineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fcf_PreTslIvMember
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:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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