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Investment Securities - Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items] |  |  |
Maturity of contract | less than one year |  |
Approximate year of maturity | 30 years |  |
Gross Unrealized Losses | $ 24,420,000us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss | $ 47,666,000us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss |
Amortized Cost | 1,317,318,000us-gaap_AvailableForSaleSecuritiesAmortizedCost | 1,350,466,000us-gaap_AvailableForSaleSecuritiesAmortizedCost |
Net securities gains (losses) | 550,000us-gaap_GainLossOnSaleOfSecuritiesNet | (1,158,000)us-gaap_GainLossOnSaleOfSecuritiesNet | 192,000us-gaap_GainLossOnSaleOfSecuritiesNet
Available-for-sale securities pledged as collateral | 563,200,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral | 594,900,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral |
Pooled Trust Preferred Collateralized Debt Obligations [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Gross Unrealized Losses | 500,000us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_InvestmentTypeAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember | 1,300,000us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss / us-gaap_InvestmentTypeAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |
Amortized Cost | 6,600,000us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |  |
Net securities gains (losses) | 200,000us-gaap_GainLossOnSaleOfSecuritiesNet / us-gaap_InvestmentTypeAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember |  |
Pooled Trust Preferred Collateralized Debt Obligations [Member] | Non-bank subsidiaries [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Sale of available securities | 200,000fcf_SalesOfAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_TrustPreferredSecuritiesSubjectToMandatoryRedemptionMember / dei_LegalEntityAxis = fcf_NonbankSubsidiariesMember |  |
Single issuer trust preferred securities [Member] | Non-bank subsidiaries [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Sale of available securities | $ 5,100,000fcf_SalesOfAvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = fcf_SingleIssuerTrustPreferredSecuritiesMember / dei_LegalEntityAxis = fcf_NonbankSubsidiariesMember |  |

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- Definition Sales Of Available For Sale Securities + References No definition available. + Details
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- 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
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- 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
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- Definition Description of how prepayments are considered in determination of contractual cash flows and cash flows expected to be collected for the loan or debt security. This relates to loans and debt securities for which at acquisition it was probable that the investor would be unable to collect all contractually required payments receivable or all cash flows expected at acquisition. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15029-111544 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538 + Details
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- Definition The net gain (loss) realized from the sale, exchange, redemption, or retirement of securities, not separately or otherwise categorized as trading, available-for-sale, or held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 13 -Subparagraph h -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.3) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 3 -Article 7 + Details
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