Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 09, 2018
Document and Entity Information [Abstract]    
Document Period End Date Sep. 30, 2018  
Entity Registrant Name JOUNCE THERAPEUTICS, INC.  
Entity Central Index Key 0001640455  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Small Business true  
Document Type 10-Q  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Trading Symbol JNCE  
Entity Common Stock, Shares Outstanding   32,700,495