Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Aug. 06, 2019
Document And Entity Information [Abstract]    
Entity Registrant Name Sesen Bio, Inc.  
Entity Central Index Key 0001485003  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Document Type 10-Q  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Amendment Flag false  
Entity Emerging Growth Company true  
Entity Small Business true  
Entity Transition Period Flag true  
Entity Shell Company false  
Entity Current Reporting Status Yes  
Entity Common Stock, Shares Outstanding   101,265,896