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Interest-Bearing Deposits - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Deposits [Abstract] |  |  |
Interest-bearing demand deposit, money market | $ 1,200.0 | $ 1,100.0 |
Certificates of deposit in denominations of $100 thousand or more | 93.1 | 143.9 |
Interest expense related to certificates of deposit $100 thousand or greater | 2.0 | 3.3 | $ 1.6
Deposit Liabilities Reclassified as Loans Receivable | $ 0.7 | $ 0.9 |

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- Definition Interest Expense, Time Deposits, $250,000 or More + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition TIme Deposits, $250,000 or More + References No definition available. + Details
Name: | fcf_TImeDeposits250000orMore
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of deposits held by the entity that have been reclassified as loan balances receivable, for example, but not limited to, overdrafts receivable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 405 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
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- Definition Amount of interest-bearing domestic money market demand account deposit liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.12) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant