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Interest-Bearing Deposits - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Deposits [Abstract] |  |  |
Interest-bearing demand deposit, money market | $ 1,200,000,000 |  |
Certificates of deposit in denominations of $100 thousand or more | 459,900,000 | $ 288,800,000 |
Interest expense related to certificates of deposit $100 thousand or greater | 16,600,000 | 8,100,000 | $ 200,000
Deposit Liabilities Reclassified as Loans Receivable | $ 900,000 | $ 700,000 |

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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 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -SubTopic 405 -Topic 942 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478353/942-405-50-1 + 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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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
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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 -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | credit
Period Type: | instant