| Document and Entity Information Document and Entity Information | Apr. 27, 2021 |
| Document Information [Line Items] | |
| Entity Incorporation, State or Country Code | PA |
| Amendment Flag | false |
| Entity Registrant Name | FIRST COMMONWEALTH FINANCIAL CORP /PA/ |
| Document Fiscal Year Focus | 2021 |
| Entity Address, State or Province | PA |
| Entity File Number | 001-11138 |
| Entity Tax Identification Number | 25-1428528 |
| Document Type | 8-K |
| Entity Address, Address Line One | 601 Philadelphia Street |
| Entity Address, City or Town | Indiana |
| Entity Address, Postal Zip Code | 15701 |
| Document Period End Date | Apr. 27, 2021 |
| City Area Code | 724 |
| Local Phone Number | 349-7220 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Document Fiscal Period Focus | FY |
| Entity Central Index Key | 0000712537 |
