Document and Entity Information - USD ($)
$ in Millions
12 Months Ended
Mar. 31, 2019
Jun. 21, 2019
Sep. 28, 2018
Document And Entity Information      
Entity Registrant Name REPLIMUNE GROUP, INC.    
Entity Central Index Key 0001737953    
Document Type 10-K    
Document Period End Date Mar. 31, 2019    
Amendment Flag false    
Current Fiscal Year End Date --03-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Non-accelerated Filer    
Entity Public Float     $ 184
Entity Small Business false    
Entity Shell Company false    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Common Stock, Shares Outstanding   31,663,701  
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY