v3.19.3
Cover Page - shares
9 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Current Fiscal Year End Date --12-31  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Trading Symbol MGTA  
Entity Registrant Name Magenta Therapeutics, Inc.  
Entity Central Index Key 0001690585  
Entity Filer Category Non-accelerated Filer  
Entity Shell Company false  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Common Stock, Shares Outstanding   38,989,517
Entity Ex Transition Period false  
Title of 12(b) Security Common Stock  
Security Exchange Name NASDAQ  
Entity Address, State or Province MA