| **Fair Values of Assets and Liabilities - Additional Information (Detail) (USD $)**                                              | 3 Months Ended | 6 Months Ended |             |
|----------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|-------------|
||
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                               |           |           |        |   |   |
| Fair Value, Assets, Level 1 to Level 2 Transfers, Description                                                                    | $ 0.000        |           | $ 0.000     |   |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                   | 2,319,000      |           | 3,035,000   | 20,135,000 |   |
| Impaired loans considered to be credit risk of non-collection                                                                    | 100,000        |           | 100,000     |   |   |
| Updated appraisal requirement floor                                                                                              | 250,000        |           | 250,000     |   |   |
| Other real estate owned                                                                                                          | 7,817,000      |           | 7,817,000   |   | 11,728,000 |
| Financial instrument of credit risk                                                                                              | 100,000        |           | 100,000     |   | 100,000 |
| Fair Value, Assets Measured on Recurring Basis, Gain (Loss) Included in Other Income, Assets and Liabilities Continue to be held |           | 0              | 0           | 0 |   |
| Other Assets [Member]                                                                                                            |           |           |        |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                               |           |           |        |   |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                   | 0              |           | 0           | 0 |   |
| Loans Held For Sale [Member]                                                                                                     |           |           |        |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                               |           |           |        |   |   |
| Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3                   | $ 2,319,000    |           | $ 3,035,000 | $ 20,135,000 |   |
| Minimum [Member]                                                                                                                 |           |           |        |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                               |           |           |        |   |   |
| Swap Rates Term Used to Determine Yield Curve                                                                                    |           |           | 3 years     |   |   |
| Maximum [Member]                                                                                                                 |           |           |        |   |   |
| **Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]**                               |           |           |        |   |   |
| Swap Rates Term Used to Determine Yield Curve                                                                                    |           |           | 30 years    |   |   |

| X                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Assets Measured on Recurring Basis, Gain (Loss) Included in Other Income, Assets and Liabilities Continue to be held
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Impaired loans fair value non collection.
+ References
No definition available.
+ Details**Name:**fcf_ImpairedLoansFairValueNonCollection**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Swap Rates Term Used to Determine Yield Curve
+ References
No definition available.
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| X                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Updated appraisal requirement floor.
+ References
No definition available.
+ Details**Name:**fcf_UpdatedAppraisalRequirementFloor**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
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| X                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents disclosure of all significant concentrations of credit risk or market risk arising from the subject financial instrument (as defined), whether from an individual counterparty or groups of counterparties.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 25 -Article 5Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 7Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 17 -Article 9
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5
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