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v3.19.1

Subsequent Event Subsequent Event Table (Details) $ in Millions | Mar. 31, 2019 USD ($)
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Subsequent Event [Line Items] |
Branch Acquisition, Number of Branches | 14
Branch Acquisition, Total Deposits | $ 525
Branch Acquisition, Total Loans | $ 120

X
- Definition Branch Acquisition, Number of Branches + References No definition available. + Details
Name: | fcf_BranchAcquisitionNumberofBranches
Namespace Prefix: | fcf_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Branch Acquisition, Total Deposits + References No definition available. + Details
Name: | fcf_BranchAcquisitionTotalDeposits
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Branch Acquisition, Total Loans + References No definition available. + Details
Name: | fcf_BranchAcquisitionTotalLoans
Namespace Prefix: | fcf_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration