| **Impairment of Investment Securities - Pooled Trust Preferred Collateralized Debt Obligations (Detail) (USD $)In Thousands, unless otherwise specified** |           |           |        | 3 Months Ended |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------|-----------|-----------|--------|----------------|
||
| **Impairment Of Investment Securities [Line Items]**                                                                                                      |      |      |   |           |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Book Value                                                                                                                                                |      |      | $ 2    | $ 1,219        | $ 1,830 | $ 55 | $ 3,686 | $ 1,868 | $ 2,277 | $ 1,343 | $ 5,400 | $ 12,719 | $ 13,304 | $ 867 | $ 6,507 | $ 51,077 |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Estimated Fair Value                                                                                                                                      | 1,297,203 | 1,171,303 | 2      | 1,203          | 1,160 | 56 | 3,513 | 1,007 | 1,054 | 1,157 | 2,553 | 5,561 | 5,230 | 520 | 1,496 | 24,512 |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Unrealized Gain (Loss)                                                                                                                                    |      |      | $ 0    | $ (16)         | $ (670) | $ 1 | $ (173) | $ (861) | $ (1,223) | $ (186) | $ (2,847) | $ (7,158) | $ (8,074) | $ (347) | $ (5,011) | $ (26,565) |   |   |   |   |   |   |   |   |   |   |   |   |   |
| Number of Banks (in banks)                                                                                                                                |      |      |   |           |   |   |   |   |   |   |   |   |   |   |   |   | 3 | 9 | 16 | 6 | 31 | 16 | 45 | 48 | 70 | 63 | 61 | 16 | 28 |
| Deferrals and Defaults as a % of Current Collateral                                                                                                       |      |      |   |           |   |   |   |   |   |   |   |   |   |   |   |   | 100.00% | 33.33% | 54.75% | 27.07% | 54.03% | 45.70% | 27.04% | 34.74% | 32.55% | 31.20% | 36.03% | 54.75% | 43.38% |
| Excess Subordination as a % of Current Performing Collateral                                                                                              |      |      |   |           |   |   |   |   |   |   |   |   |   |   |   |   | 0.00% |   | 631.19% | 80.70% | 0.00% | 0.00% | 10.57% | 0.00% | 0.00% | 15.98% | 31.67% | 0.00% | 0.00% |

| X                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Available for sale securities book value.
+ References
No definition available.
+ Details**Name:**fcf_AvailableForSaleSecuritiesBookValue**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Deferrals and defaults as a percentage of current collateral.
+ References
No definition available.
+ Details**Name:**fcf_DeferralsAndDefaultsAsPercentageOfCurrentCollateral**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Excess subordination as a percentage of current performing collateral.
+ References
No definition available.
+ Details**Name:**fcf_ExcessSubordinationAsPercentageOfCurrentPerformingCollateral**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_ImpairmentOfInvestmentSecuritiesLineItems**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Pooled trust preferred collateralized debt obligations in number of banks.
+ References
No definition available.
+ Details**Name:**fcf_PooledTrustPreferredCollateralizedDebtObligationsInNumberOfBanks**Namespace Prefix:**fcf_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.
+ 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=7534914&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net amount of gross unrealized gains (losses) for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossUnrealizedGainLoss**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |