Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2014	Mar. 31, 2012	Dec. 31, 2013
Investment Securities [Line Items]
Amortized Cost	$ 1,370,681		$ 1,350,466
Gross Unrealized Gains	16,068	15,565
Gross Unrealized Losses	(35,710)	(47,666)
Securities available for sale, at fair value	(1,351,039)		(1,318,365)
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	21,648		22,639
Gross Unrealized Gains	2,627	2,624
Gross Unrealized Losses	34	(59)
Securities available for sale, at fair value	(24,241)		(25,204)
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	1,020,102		1,009,519
Gross Unrealized Gains	12,977	12,531
Gross Unrealized Losses	19,472	(27,163)
Securities available for sale, at fair value	(1,013,607)		(994,887)
Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]
Investment Securities [Line Items]
Amortized Cost	96		104
Gross Unrealized Gains	2	1
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	(98)		(105)
Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	269,170		267,971
Gross Unrealized Gains	3	81
Gross Unrealized Losses	1,863	(1,927)
Securities available for sale, at fair value	(267,310)		(266,125)
Obligations of States and Political Subdivisions [Member]
Investment Securities [Line Items]
Amortized Cost	9,340		80
Gross Unrealized Gains	22	0
Gross Unrealized Losses	3	0
Securities available for sale, at fair value	(9,359)		(80)
Corporate Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	6,691		6,693
Gross Unrealized Gains	437	328
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	(7,128)		(7,021)
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Investment Securities [Line Items]
Amortized Cost	42,214		42,040
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(14,338)	(18,517)
Securities available for sale, at fair value	(27,876)		(23,523)
Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,369,261		1,349,046
Gross Unrealized Gains	16,068	15,565
Gross Unrealized Losses	(35,710)	(47,666)
Securities available for sale, at fair value	(1,349,619)		(1,316,945)
Equity Securities [Member]
Investment Securities [Line Items]
Amortized Cost	1,420		1,420
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Securities available for sale, at fair value	$ (1,420)		$ (1,420)

Name:	fcf_InvestmentSecuritiesLineItems
Namespace Prefix:	fcf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration