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Subordinated Debentures - Additional Information (Detail) (USD $)	12 Months Ended	0 Months Ended	12 Months Ended
	Dec. 31, 2014	Apr. 02, 2013	Dec. 31, 2013
First Commonwealth Capital Trust [Member]
Subordinated Borrowing [Line Items]
Percentage of common equity owned by First Commonwealth	100.00%fcf_PercentageOfEachTrustCommonEquityOwnedByCorporation / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustMember
First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Redemption price as percentage of principal amount	100.00%fcf_RedemptionPriceAsPercentageOfPrincipalAmount / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustThreeMember
Deferred finance costs	$ 630,000us-gaap_DeferredFinanceCostsGross / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustThreeMember
First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Redemption price as percentage of principal amount	100.00%fcf_RedemptionPriceAsPercentageOfPrincipalAmount / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustTwoMember
Deferred finance costs	471,000us-gaap_DeferredFinanceCostsGross / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustTwoMember
First Commonwealth Capital Trust I [Member]
Subordinated Borrowing [Line Items]
Deferred issuance costs		500,000us-gaap_WriteOffOfDeferredDebtIssuanceCost / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember
Debentures authorized for redemption percentage		100.00%fcf_PercentageOfPrincipalAmountAuthorizedForRedemptionChange / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember
Redemption price as percentage of principal amount		103.325%fcf_RedemptionPriceAsPercentageOfPrincipalAmount / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember
Deferred finance costs	996,000us-gaap_DeferredFinanceCostsGross / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember
Redemption Premium		$ 1,100,000fcf_PaymentofRedemptionPremium / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember
Fixed interest rate		9.50%us-gaap_SubordinatedBorrowingInterestRate / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Capital Trust III [Member]
Subordinated Borrowing [Line Items]
Variable interest rate	2.85%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustThreeMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember		2.85%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustThreeMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Capital Trust II [Member]
Subordinated Borrowing [Line Items]
Variable interest rate	2.85%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustTwoMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember		2.85%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_LongtermDebtTypeAxis = us-gaap_SubordinatedDebtMember / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustTwoMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember

X

- Definition

Payment of Redemption Premium

+ References

No definition available.

+ Details

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

X

- Definition

Percentage Of Each Trust Common Equity Owned By Corporation

+ References

No definition available.

+ Details

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

X

- Definition

Percentage of outstanding principal available to be redeemed.

+ References

No definition available.

+ Details

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

X

- Definition

Redemption Price As Percentage Of Principal Amount

+ References

No definition available.

+ Details

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Namespace Prefix:	fcf_
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Balance Type:	na
Period Type:	duration

X

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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

X

- Definition

For an unclassified balance sheet, the amount as of the balance sheet date of capitalized costs associated with the issuance of debt instruments (for example, legal, accounting, underwriting, printing, and registration costs) that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Stated interest rate of the subordinated debt.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_SubordinatedBorrowingLineItems
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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Write-off of amounts previously capitalized as debt issuance cost in an extinguishment of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.8)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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Balance Type:	debit
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- Details

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X

- Details

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- Details

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- Details

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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
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