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Cover Page - shares
3 Months Ended
Mar. 31, 2024
May 08, 2024
Cover [Abstract]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Mar. 31, 2024  
Document Fiscal Year Focus 2024  
Document Transition Report false  
Entity File Number 001-39319  
Entity Registrant Name GENERATION BIO CO.  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 81-4301284  
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  
Title of 12(b) Security Common Stock, $0.0001 Par Value  
Trading Symbol GBIO  
Security Exchange Name 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 false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding   66,528,840
Entity Central Index Key 0001733294  
Current Fiscal Year End Date --12-31  
Document Fiscal Period Focus Q1  
Amendment Flag false