| Cover | Jan. 10, 2022 |
| Cover [Abstract] | |
| Document Period End Date | Jan. 10, 2022 |
| Document Type | 8-K |
| Entity Registrant Name | Foghorn Therapeutics Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-39634 |
| Entity Tax Identification Number | 47-5271393 |
| Entity Address, Address Line One | 500 Technology Square, Ste 700 |
| Entity Address, Postal Zip Code | 02139 |
| City Area Code | 617 |
| Local Phone Number | 586-3100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of each class | Common Stock, $0.0001 par value per share |
| Trading Symbol(s) | FHTX |
| Name of each exchange on which registered | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001822462 |
| Entity Address, City or Town | MA |
| Entity Address, City or Town | Cambridge, |
