| **Investment Securities - Analysis of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $)In Thousands, unless otherwise specified** | 6 Months Ended |          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------|
||
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | $ 1,357,315    |     | $ 1,350,466 |
| Gross Unrealized Gains                                                                                                                                                     | 18,717         | 15,565   |   |
| Gross Unrealized Losses                                                                                                                                                    | (28,421)       | (47,666) |   |
| Securities available for sale, at fair value                                                                                                                               | 1,347,611      |     | 1,318,365 |
| US Government Agencies Residential Mortgage-backed Securities [Member]                                                                                                     |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 20,686         |     | 22,639 |
| Gross Unrealized Gains                                                                                                                                                     | 2,617          | 2,624    |   |
| Gross Unrealized Losses                                                                                                                                                    | 0              | (59)     |   |
| Securities available for sale, at fair value                                                                                                                               | 23,303         |     | 25,204 |
| Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Residential [Member]                                                                   |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 1,001,669      |     | 1,009,519 |
| Gross Unrealized Gains                                                                                                                                                     | 15,257         | 12,531   |   |
| Gross Unrealized Losses                                                                                                                                                    | 13,807         | (27,163) |   |
| Securities available for sale, at fair value                                                                                                                               | 1,003,119      |     | 994,887 |
| Obligations of U.S. Government-Sponsored Enterprises - Mortgage-Backed Securities - Commercial [Member]                                                                    |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 88             |     | 104 |
| Gross Unrealized Gains                                                                                                                                                     | 2              | 1        |   |
| Gross Unrealized Losses                                                                                                                                                    | 0              | 0        |   |
| Securities available for sale, at fair value                                                                                                                               | 90             |     | 105 |
| Obligations of U.S. Government-Sponsored Enterprises - Other Government-Sponsored Enterprises [Member]                                                                     |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 269,175        |     | 267,971 |
| Gross Unrealized Gains                                                                                                                                                     | 15             | 81       |   |
| Gross Unrealized Losses                                                                                                                                                    | 803            | (1,927)  |   |
| Securities available for sale, at fair value                                                                                                                               | 268,387        |     | 266,125 |
| Obligations of States and Political Subdivisions [Member]                                                                                                                  |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 15,738         |     | 80 |
| Gross Unrealized Gains                                                                                                                                                     | 156            | 0        |   |
| Gross Unrealized Losses                                                                                                                                                    | 5              | 0        |   |
| Securities available for sale, at fair value                                                                                                                               | 15,889         |     | 80 |
| Corporate Debt Securities [Member]                                                                                                                                         |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 6,688          |     | 6,693 |
| Gross Unrealized Gains                                                                                                                                                     | 561            | 328      |   |
| Gross Unrealized Losses                                                                                                                                                    | 0              | 0        |   |
| Securities available for sale, at fair value                                                                                                                               | 7,249          |     | 7,021 |
| Pooled Trust Preferred Collateralized Debt Obligations [Member]                                                                                                            |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 41,851         |     | 42,040 |
| Gross Unrealized Gains                                                                                                                                                     | 109            | 0        |   |
| Gross Unrealized Losses                                                                                                                                                    | (13,806)       | (18,517) |   |
| Securities available for sale, at fair value                                                                                                                               | 28,154         |     | 23,523 |
| Debt Securities [Member]                                                                                                                                                   |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 1,355,895      |     | 1,349,046 |
| Gross Unrealized Gains                                                                                                                                                     | 18,717         | 15,565   |   |
| Gross Unrealized Losses                                                                                                                                                    | (28,421)       | (47,666) |   |
| Securities available for sale, at fair value                                                                                                                               | 1,346,191      |     | 1,316,945 |
| Equity Securities [Member]                                                                                                                                                 |           |     |   |
| **Investment Securities [Line Items]**                                                                                                                                     |           |     |   |
| Amortized Cost                                                                                                                                                             | 1,420          |     | 1,420 |
| Gross Unrealized Gains                                                                                                                                                     | 0              | 0        |   |
| Gross Unrealized Losses                                                                                                                                                    | 0              | 0        |   |
| Securities available for sale, at fair value                                                                                                                               | $ 1,420        |     | $ 1,420 |

| X                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_InvestmentSecuritiesLineItems**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| 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
+ Details**Name:**us-gaap_AvailableForSaleSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.
+ 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 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesAmortizedCost**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGains**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |