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v3.8.0.1

Retirement Plans - Additional Information (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Employer contribution | 6.00% |  |
Employee contribution | 80.00% |  |
Plan expense | $ 2,800,000 | $ 2,500,000 | $ 2,700,000
Retirement Plan expense | 0 | $ 0 |
Minimum [Member] |  |  |
Total annual compensation for a calendar year for non-qualified deferred compensation plan | $ 110,000 |  |
Eligibility of participants | 1.00% |  |
Maximum [Member] |  |  |
Eligibility of participants | 25.00% |  |

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- Definition Deferred Compensation Plan Net Of Contribution Expenses + 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 Participant Contribution On Pre Tax Basis Under Retirement Plan + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage Of Employees Eligible Compensation That Represents Maximum Amount Of Employer Matching Contributions To Four Zero One K Plan + References No definition available. + Details
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Namespace Prefix: | fcf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Retirement Plan Expense + 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 Total Annual Compensation for a Calendar Year for Non-Qualified Deferred Compensation Plan + 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 Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |