| **Statement of Accounting Policies - Additional Information (Detail) (USD $)**        | 12 Months Ended |             |             | 12 Months Ended |
|---------------------------------------------------------------------------------------|-----------------|-------------|-------------|-----------------|
||
| **Summary Of Significant Accounting Policies [Line Items]**                           |            |        |        |            |
| Business segments                                                                     | 1               |        |        |            |
| Equity investments                                                                    | 20.00%          |        |        |            |
| Loans past due days                                                                   | 90 days         |        |        |            |
| Excess impaired credits                                                               | $ 100,000       |        |        |            |
| Other liabilities                                                                     |            | $ 3,600,000 | $ 3,800,000 |            |
| Software purchases and developed are capitalized and amortized on straight-line basis | 7 years         |        |        | 10 years        |

| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Dollar threshold in which impaired credits are individually evaluated and reviewed.
+ References
No definition available.
+ Details**Name:**fcf_ExcessImpairedLoanCredit**Namespace Prefix:**fcf_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Loans Past Due Days
+ References
No definition available.
+ Details**Name:**fcf_LoansPastDueDays**Namespace Prefix:**fcf_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Equity investments in which ownership is at least 20% are accounted for using the "equity method" of accounting and are classified as 'Other Assets".
+ References
No definition available.
+ Details**Name:**fcf_PercentageOfOwnershipInEquityInvestments**Namespace Prefix:**fcf_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_SummaryOfSignificantAccountingPoliciesLineItems**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Useful life of capitalized software amortized on a straight-line basis.
+ References
No definition available.
+ Details**Name:**fcf_UsefulLifeOfCapitalizedSoftware**Namespace Prefix:**fcf_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The sum of the known and estimated amounts payable as of the balance sheet date to policyholders pertaining to insured events for long-duration contracts, which can be viewed as either (a) the present value of future benefits to be paid to or on behalf of policyholders and expenses less the present value of future net premiums payable under the insurance contracts or (b) the accumulated amount of net premiums already collected less the accumulated amount of benefits and expenses already paid to or on behalf of policyholders.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 25 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6482067&loc=d3e13145-158430Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph a -Article 7Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.13(a)(1)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 60 -Paragraph 21-26 -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 97 -Paragraph 15-21 -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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 113 -Paragraph 26 -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.
+ Details**Name:**us-gaap_LiabilityForFuturePolicyBenefits**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.
+ References
No definition available.
+ Details**Name:**us-gaap_NumberOfOperatingSegments**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**duration |