| **Goodwill and Other Amortizing Intangible Assets** | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Goodwill Disclosure [Abstract]**                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Goodwill and Other Amortizing Intangible Assets     | Goodwill and Other Amortizing Intangible AssetsFASB ASC Topic 350-20, “Intangibles—Goodwill and Other,” requires an annual valuation of the fair value of a reporting unit that has goodwill and a comparison of the fair value to the book value of equity to determine whether the goodwill has been impaired. Goodwill is also required to be tested on an interim basis if an event or circumstance indicates that it is more likely than not that an impairment loss has been incurred. When triggering events or circumstances indicate goodwill testing is required, an assessment of qualitative factors can be completed before performing the two step goodwill impairment test. ASU 2011-8 provides that if an assessment of qualitative factors determines it is more likely than not that the fair value of a reporting unit exceeds its carrying amount, then the two step goodwill impairment test is not required.We consider First Commonwealth to be one reporting unit. The carrying amount of goodwill as of December 31, 2014 and 2013 was $161.4 million and $159.9 million, respectively. No impairment charges on goodwill or other intangible assets were incurred in 2014, 2013 or 2012.We test goodwill for impairment as of November 30th each year and again at any quarter-end if any material events occur during a quarter that may affect goodwill.An assessment of qualitative factors was completed as of November 30, 2014 and December 31, 2014 and indicated that it is more likely than not that the fair value of First Commonwealth's goodwill exceeds its carrying amount; therefore, the two step goodwill impairment test was not considered necessary. The assessment of qualitative factors considered the results of the Step 1 goodwill impairment test completed as of November 30, 2013 as well as macroeconomic factors, industry and market considerations, the company’s overall financial performance, and other company specific events occurring since the completion of the November 30, 2013 test.As of December 31, 2014, goodwill was not considered impaired; however, changing economic conditions that may adversely affect our performance, the fair value of our assets and liabilities, our stock price could result in impairment, which could adversely affect earnings in future periods. Management will continue to monitor events that could impact this conclusion in the future.FASB ASC Topic 350, “Intangibles—Other,” also requires that an acquired intangible asset be separately recognized if the benefit of the intangible asset is obtained through contractual or other legal rights, or if the asset can be sold, transferred, licensed, rented or exchanged, regardless of the acquirer’s intent to do so.The following table summarizes other intangible assets, which for each year includes only core deposit intangibles: GrossIntangibleAssets AccumulatedAmortization NetIntangibleAssets (dollars in thousands)December 31, 2014     Customer deposit intangibles$22,470 $(21,773) $697Customer list intangible$984 $(16) $968Total other intangible assets$23,454 $(21,789) $1,665      December 31, 2013     Customer deposit intangibles$22,470 $(21,159) $1,311Customer list intangible$— $— $—Total other intangible assets$22,470 $(21,159) $1,311Core deposits are amortized over their expected lives using the present value of the benefit of the core deposits and straight-line methods of amortization. The core deposits have a remaining amortization period of 4.9 years and a weighted average amortization period of approximately three years. The customer list intangible represents the estimated value of the customer base  for an insurance agency acquired in 2014.  These amounts are amortized over their expected lives using expected cash flows based on retention of the customer base.  The customer list intangible has a remaining amortization period and a weighted average amortization period of 14.8 years. First Commonwealth recognized amortization expense on other intangible assets of $0.6 million, $1.1 million, and $1.5 million for the years ended December 31, 2014, 2013 and 2012, respectively.The following presents the estimated amortization expense of core deposit intangibles and the customer intangible list: Core Deposit IntangiblesCustomer Intangible ListTotal (dollars in thousands)2015$338$260$59820161771683452017631301932018629916120195777134Thereafter—234234Total$697$968$1,665 |

| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**fcf_GoodwillDisclosureAbstract**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The entire disclosure for the aggregate amount of goodwill and a description of intangible assets, which may include (a) for amortizable intangible assets (also referred to as finite-lived intangible assets), the carrying amount, the amount of any significant residual value, and the weighted-average amortization period, (b) for intangible assets not subject to amortization (also referred to as indefinite-lived intangible assets), the carrying amount, and (c) the amount of research and development assets acquired and written off in the period, including the line item in the income statement in which the amounts written off are aggregated, if not readily apparent from the income statement. Also discloses (a) for amortizable intangibles assets in total and by major class, the gross carrying amount and accumulated amortization, the total amortization expense for the period, and the estimated aggregate amortization expense for each of the five succeeding fiscal years, (b) for intangible assets not subject to amortization the carrying amount in total and by major class, and (c) for goodwill, in total and for each reportable segment, the changes in the carrying amount of goodwill during the period (including the aggregate amount of goodwill acquired, the aggregate amount of impairment losses recognized, and the amount of goodwill included in the gain (loss) on disposal of a reporting unit). If any part of goodwill has not been allocated to a reportable segment, discloses the unallocated amount and the reasons for not allocating. For each impairment loss recognized related to an intangible asset (excluding goodwill), discloses: (a) a description of the impaired intangible asset and the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method for determining fair value, (c) the caption in the income statement or the statement of activities in which the impairment loss is aggregated, and (d) the segment in which the impaired intangible asset is reported. For each goodwill impairment loss recognized, discloses: (a) a description of the facts and circumstances leading to the impairment, (b) the amount of the impairment loss and the method of determining the fair value of the associated reporting unit, and (c) if a recognized impairment loss is an estimate not finalized and the reasons why the estimate is not final.  May also disclose the nature and amount of any significant adjustments made to a previous estimate of an impairment loss.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16265-109275Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16373-109275Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13854-109267
+ Details**Name:**us-gaap_GoodwillAndIntangibleAssetsDisclosureTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |