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Commitments and Letters of Credit - Notional Amount of Commitments and Letter of Credit Instruments (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Loss Contingencies [Line Items] |  |  |
Commitments Net | $ 71 | $ 223 | $ 1,087
Commitments to extend credit |  |  |
Loss Contingencies [Line Items] |  |  |
Financial instrument of credit risk | 2,097,628 | 1,981,275 |
Financial standby letters of credit |  |  |
Loss Contingencies [Line Items] |  |  |
Financial instrument of credit risk | 15,988 | 16,630 |
Performance standby letters of credit |  |  |
Loss Contingencies [Line Items] |  |  |
Financial instrument of credit risk | 16,864 | 23,293 |
Commercial letters of credit |  |  |
Loss Contingencies [Line Items] |  |  |
Financial instrument of credit risk | $ 783 | $ 783 |

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- Definition Total commitments. + 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 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
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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: |
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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
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