| **Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) (USD $)**                                                                                                                                                                  |                 |                 | 6 Months Ended |        | 6 Months Ended |        | 6 Months Ended |         | 6 Months Ended |        |        |        |        |     |        |     |        |     |        |     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|-----------------|----------------|--------|----------------|--------|----------------|---------|----------------|--------|--------|--------|--------|-----|--------|-----|--------|-----|--------|-----|
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| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]**                                                                                                                                                                                                          |            |            |           |   |                |   |           |    |           |   |   |   |   |     |   |     |   |     |   |     |   |  |   |  |   |  |   |   |  |   |   |  |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Securities available for sale, at fair value                                                                                                                                                                                                                                                              | $ 1,347,611,000 | $ 1,318,365,000 |           |   |                |   |           |    |           |   |   |   |   |     |   |     |   |     |   |     |   |  |   |  |   |  |   |   |  |   |   |  | $ 29,574,000 | $ 24,943,000 | $ 1,420,000 | $ 1,420,000 | $ 28,154,000 | $ 23,523,000 | $ 23,523,000 | $ 28,154,000 |   |   |   |   |   |
| Other Real Estate Owned Assets Fair Value                                                                                                                                                                                                                                                                 |            |            |           |   |                |   |           |    |           |   |   |   |   |     |   |     |   |     |   |     |   |  |   |  |   |  |   |   |  |   |   |  |   |   |   |   |   |   |   |   | 8,360,000 | 12,924,000 | 139,000 | 172,000 | 139,000 |
| Assets                                                                                                                                                                                                                                                                                                    |            |            |           |   |                |   |           |    |           |   |   | 0      |   |     |   |     |   |     |   |     |   |  |   |  |   |  | 6,120,000 |   |  | 113,000 |   |  | 29,574,000 | 24,943,000 |   |   |   |   |   |   | 48,575,000 | 63,483,000 | 13,012,000 | 13,828,000 |   |
| Weighted average rate                                                                                                                                                                                                                                                                                     |            |            | 5.50%          | 14.50% | [1]            | 0.00%  | 0.00%          | 100.00% | 100.00%        | 17.48% | 6.35%  |   | 3.74%  | [2] | 6.62%  | [2] |   |     |   |     |   |  |   |  |   |  |   | 10.00% | [2] |   | 9.00% | [2] |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Fair Value Inputs Monetary                                                                                                                                                                                                                                                                                |            |            |           |   |                |   |           |    |           |   |   |   |   |     |   |     | $ 0.83 | [3] | $ 3.84 | [3] | $ 83.64 | [3] | $ 6.45 | [3] | $ 107.00 | [3] |   |   |  |   |   |  |   |   |   |   |   |   |   |   |   |   |   |   |   |
|                                                                                                                                                                                                                                                                                                           |
| [1](a)incorporates spread over risk free rate related primarily to credit quality and illiquidity of secu[2]represents the range of the credit spread curve used in valuation.[3]unobservable inputs are defined as follows: MCF - million cubic feet; BBL/d - barrels per day; NGL - natural gas liquid. |

| X                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value Weighted Average Inputs Price
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other real estate owned assets fair value.
+ References
No definition available.
+ Details**Name:**fcf_OtherRealEstateOwnedAssetsFairValue**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
+ 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 (a) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
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| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value.  For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.
+ 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 (bbb) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258
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