Cover Page - shares	3 Months Ended
	Mar. 31, 2022	Apr. 29, 2022
Cover [Abstract]
Document Type	10-Q/A
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2022
Document Transition Report	false
Entity File Number	001-39634
Entity Registrant Name	Foghorn Therapeutics Inc.
Entity Incorporation, State	DE
Entity Tax Identification Number	47-5271393
Entity Address, Street Name	500 Technology Square, Ste 700
Entity Address, City	Cambridge
Entity Address, State	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	586-3100
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		41,467,808
Amendment Flag	true
Document Fiscal Year Focus	2022
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001822462
Current Fiscal Year End Date	--12-31
Amendment Description	EXPLANATORY NOTEFoghorn Therapeutics Inc. (the “Company”) is filing this Amendment No. 1 (this “Amendment”) on Form 10-Q/A to amend its Quarterly Report on Form 10-Q for the quarter ended March 31, 2022, filed with the Securities and Exchange Commission on May 9, 2022 (the “Original 10-Q”), for the purpose of filing revised versions of Exhibits 31.1 and 31.2 filed with the Original 10-Q.The Company is filing revised exhibits solely in order to include in the certifications set forth in the Exhibits the language added to the introductory portion of paragraph 4 and the language of revised paragraph 4(b), which language was inadvertently omitted from the certifications when originally filed. This Amendment does not reflect events occurring after the date of the filing of the Original 10-Q or modify or update any of the other disclosures contained therein in any way. Accordingly, this Amendment should be read in conjunction with the Original 10-Q. This Amendment consists solely of the preceding cover page, this Explanatory Note, the signature page and paragraphs 1, 2, 4 and 5 of each of the revised certifications filed as exhibits to the Amendment. Because no financial statements have been include in this Amendment, paragraph 3 of each of the certifications set forth in the Exhibits has been omitted.