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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, 2017 | Dec. 31, 2016
Not Designated as Hedging Instrument [Member] |  |
Derivative [Line Items] |  |
Credit value adjustment | $ (791) | $ (317)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 401,304 | 345,102
Not Designated as Hedging Instrument [Member] | Interest Rate Cap [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 46,444 | 14,762
Not Designated as Hedging Instrument [Member] | Credit Default Swap, Buying Protection [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 197,660 | 174,213
Not Designated as Hedging Instrument [Member] | Credit Default Swap, Selling Protection [Member] |  |
Derivative [Line Items] |  |
Derivative, Sold Protection Notional Amount | (46,170) | (40,281)
Designated as Hedging Instrument [Member] |  |
Derivative [Line Items] |  |
Fair value adjustment | 459 | (443)
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 150,000 | 200,000
Designated as Hedging Instrument [Member] | Forward Contracts [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 17,000 | 0
Fair value adjustment | 19 | 0
Designated as Hedging Instrument [Member] | Foreign Exchange Contract [Member] |  |
Derivative [Line Items] |  |
Derivative, notional amount | 10,077 | 4,749
Fair value adjustment | $ (70) | $ (8)

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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=84234895&loc=SL5624171-113959 + Details
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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: |
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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
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- Details
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Namespace Prefix: |
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- Details
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Balance Type: |
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- Details
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Balance Type: |
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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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