Cover Page - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Feb. 25, 2022	Jun. 30, 2021
Cover [Abstract]
Document Type	10-K
Document Annual Report	true
Current Fiscal Year End Date	--12-31
Document Period End Date	Dec. 31, 2021
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	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
Trading Symbol(s)	FHTX
Name of each exchange on which registered	NASDAQ
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
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
ICFR Auditor Attestation Flag	false
Entity Shell Company	false
Entity Public Float			$ 208.8
Entity Common Stock, Shares Outstanding		41,421,964
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	FY
Entity Central Index Key	0001822462