Document and Entity Information - shares
9 Months Ended
Sep. 30, 2018
Nov. 07, 2018
Document And Entity Information [Abstract]    
Trading Symbol SESN  
Entity Registrant Name Sesen Bio, Inc.  
Entity Central Index Key 0001485003  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Document Type 10-Q  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q3  
Amendment Flag false  
Entity Emerging Growth Company true  
Entity Small Business true  
Entity Transition Period Flag true  
Entity Common Stock, Shares Outstanding   77,456,180