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Derivatives - Credit Value Adjustment Recorded Related to Notional Amount of Derivatives Outstanding (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015
Not Designated as Hedging Instrument [Member] |  |
Derivative [Line Items] |  |
Credit value adjustment | $ (317) | $ (542)
Not Designated as Hedging Instrument [Member] | Interest rate swap [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 345,102 | 276,860
Not Designated as Hedging Instrument [Member] | Interest Rate Cap [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 14,762 | 22,793
Not Designated as Hedging Instrument [Member] | Credit Default Swap, Buying Protection [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 174,213 | 126,612
Not Designated as Hedging Instrument [Member] | Credit Default Swap, Selling Protection [Member] |  |
Derivative [Line Items] |  |
Derivative, Sold Protection Notional Amount | (40,281) | (20,383)
Designated as Hedging Instrument [Member] |  |
Derivative [Line Items] |  |
Fair value adjustment | (443) | 922
Designated as Hedging Instrument [Member] | Interest rate swap [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | $ 200,000 | $ 200,000

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- Definition Derivative, Sold Protection Notional Amount + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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- Definition Amount of credit risk valuation adjustment to derivative liabilities to properly reflect the credit quality of the entity. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959 + Details
Name: | us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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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: |

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