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Subordinated Debentures - Subordinated Debentures Outstanding (Detail) (USD $) In Thousands, unless otherwise specified | 0 Months Ended | 12 Months Ended
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Apr. 02, 2013 | Dec. 31, 2014 | Dec. 31, 2013
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount |  | $ 72,167us-gaap_SubordinatedDebt | 72,167us-gaap_SubordinatedDebt
First Commonwealth Capital Trust I [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Fixed interest rate | 9.50%us-gaap_SubordinatedBorrowingInterestRate / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustOneMember |  |
First Commonwealth Capital Trust II [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Due |  | Jan. 01, 2034 |
Subordinated Debentures outstanding, Amount |  | 30,929us-gaap_SubordinatedDebt / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustTwoMember | 30,929us-gaap_SubordinatedDebt / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustTwoMember
First Commonwealth Capital Trust III [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Due |  | Jan. 01, 2034 |
Subordinated Debentures outstanding, Amount |  | $ 41,238us-gaap_SubordinatedDebt / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustThreeMember | 41,238us-gaap_SubordinatedDebt / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = fcf_FirstCommonwealthCapitalTrustThreeMember
Subordinated Debt [Member] | LIBOR [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
Subordinated Debt [Member] | LIBOR [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

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- 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

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- Definition Date the payments are due on subordinated debt, in CCYY-MM-DD format. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

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- 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

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- Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 10 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Article 5 -Subsection 19, 20, 22 + Details
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Balance Type: | credit
Period Type: | instant

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- Details
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- Details
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Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |