Cover Page - shares	6 Months Ended
	Jun. 30, 2021	Jul. 30, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q2
Entity Interactive Data Current	Yes
Entity Central Index Key	0001733294
Current Fiscal Year End Date	--12-31
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Jun. 30, 2021
Entity Registrant Name	GENERATION BIO CO.
Trading Symbol	GBIO
Entity File Number	001-39319
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	81-4301284
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Address, Address Line One	301 Binney Street
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02142
City Area Code	617
Local Phone Number	655-7500
Security Exchange Name	NASDAQ
Title of 12(b) Security	Common Stock, $0.0001 Par Value
Entity Common Stock, Shares Outstanding		56,780,584
Entity Ex Transition Period	false