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Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2019 | Mar. 31, 2018
Investments, Debt and Equity Securities [Abstract] |  |
Proceeds from Sale of Available-for-sale Securities | $ 0 | $ 0
Sales Transactions: |  |
Available-for-sale Securities, Gross Realized Gains | 0 | 0
Gross losses | 0 | 0
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments | 0 | 0
Maturities and impairment |  |
Gross Gains | 0 | 2,840
Gross Losses | 0 | 0
Gain Losses Maturities And Impairment | 0 | 2,840
Net gains and impairment | $ 0 | $ 2,840

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- Definition Gain losses maturities and impairment. + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition This item represents the gross gain on maturities of available for sale securities. + References No definition available. + Details
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- Definition This item represents the gross loss on maturities of available for sale securities. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Maturities and impairment. + References No definition available. + Details
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- Definition Net gains and impairment. + References No definition available. + Details
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- 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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- Definition This item represents the gross loss 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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Balance Type: | debit
Period Type: | duration

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- Definition This item represents the difference between the gross realized gains and losses 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_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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- References No definition available. + Details
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- 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 + Details
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