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Investment Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-sale Securities, Gross Realized Gains	$ 0	$ 0
Gross Losses	0	0
Gross Gains	0	2,840
Debt Securities, Held-to-maturity, Restricted	324,700		$ 250,300
Proceeds from Sale of Available-for-sale Securities	0	$ 0
Debt Securities, Available-for-sale, Restricted	$ 625,200		$ 636,300

X

- Definition

This item represents the gross gain on maturities of available for sale securities.

+ References

No definition available.

+ Details

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

X

- Definition

This item represents the gross loss on maturities of available for sale securities.

+ References

No definition available.

+ Details

Name:	fcf_GrossLosses
Namespace Prefix:	fcf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of restricted investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 10
-Section S99
-Paragraph 3
-Subparagraph (SX 210.6-03(f))
-URI http://asc.fasb.org/extlink&oid=6488002&loc=d3e600178-122990

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X

- References

No definition available.

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Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Definition

The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26853-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-Subparagraph (a),(b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3179-108585

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