Derivatives - Credit Value Adjustment Recorded Related to Notional Amount of Derivatives Outstanding (Detail) - USD ($) $ in Thousands	12 Months Ended
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

Derivative, Sold Protection Notional Amount

No definition available.

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

- 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

- 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

- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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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- Details

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- Details

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- Details

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- Details

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- Details

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- Details

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