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Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Commitments to extend credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | $ 2,270,058 | $ 2,097,628
Financial standby letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | 15,914 | 15,988
Performance standby letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | 11,233 | 16,864
Commercial letters of credit [Member] |  |
Loss Contingencies [Line Items] |  |
Financial instrument of credit risk | $ 975 | $ 783

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- Definition Notional amount of outstanding commitments and letters of credit. + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |