Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Nov. 08, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Quarterly Report true  
Document Transition Report false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q3  
Entity Registrant Name UNUM THERAPEUTICS INC.  
Entity Central Index Key 0001622229  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company true  
Entity Ex Transition Period true  
Entity Common Stock, Shares Outstanding   30,663,054
Entity Shell Company false  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity File Number 001-38443  
Entity Incorporation, State or Country Code DE  
Entity Tax Identification Number 46-5308248  
Entity Address, Address Line One 200 Cambridge Park Drive  
Entity Address, Address Line Two Suite 3100  
Entity Address, City or Town Cambridge  
Entity Address, State or Province MA  
Entity Address, Postal Zip Code 02140  
City Area Code (617)  
Local Phone Number 945-5576  
Title of 12(b) Security Common Stock, $0.001 Par Value  
Trading Symbol UMRX  
Security Exchange Name NASDAQ