| **Goodwill**                       | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Goodwill Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| Goodwill                           | GoodwillFASB 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 that 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 September 30, 2017 and December 31, 2016 was $255.5 million and $186.5 million, respectively. Goodwill increased $69.0 million during the nine months endedSeptember 30, 2017 due to $70.7 million recognized as a result of the DCB acquisition in the second quarter of 2017 and a $1.6 million decrease related to adjustments to the fair value of assets acquired as part of the branch acquisition in the fourth quarter of 2016. No impairment charges on goodwill or other intangible assets were incurred in 2017 or 2016. 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. As of September 30, 2017, goodwill was not considered impaired; however, changing economic conditions that may adversely affect our performance, the fair value of our assets and liabilities, or 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. |

| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Goodwill Disclosure [Abstract]
+ References
No definition available.
+ Details**Name:**fcf_GoodwillDisclosureAbstract**Namespace Prefix:**fcf_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The entire disclosure for goodwill.
+ References
No definition available.
+ Details**Name:**us-gaap_GoodwillDisclosureTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |