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Revenue Recognition Revenue Recognition (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Additional Information [Abstract] |  |
Management Fees, Incentive Revenue (Deprecated 2018-01-31) | $ 0.3 |
Brokerage Commissions Revenue | $ 0.9 | $ 0.8

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- Definition Additional Information [Abstract] + References No definition available. + Details
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Balance Type: | na
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- Definition Incentive Revenue + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Amount of commission revenue from buying and selling securities on behalf of customers. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 20 -Section 05 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=68070981&loc=d3e40544-110947 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(a),(b)) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 + Details
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