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STOCKHOLDERS' EQUITY - Stock grant agreements with non-employees (Details) - Nonemployees
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Aug. 15, 2015
individual
Sep. 30, 2018
shares
Aug. 31, 2018
shares
Aug. 31, 2016
USD ($)
individual
$ / shares
shares
Aug. 31, 2015
individual
shares
Dec. 31, 2020
individual
shares
Dec. 31, 2019
individual
shares
Dec. 31, 2018
individual
shares
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Researchers, Employees of MSK                      
Share-Based Compensation Awards                      
Shares agreed to be issued         2,800,000            
Number of individuals | individual 2     2 2     2      
Issuance of common stock to nonemployees (in shares)     448,000         448,000 448,000 448,000 560,000
Shares repurchased and retired       83,600              
Estimated fair value of stock repurchased and retired (in dollars per share) | $ / shares       $ 4.38              
Reduction in equity | $       $ 366              
Researchers, Employees of MSK | Forecast                      
Share-Based Compensation Awards                      
Number of individuals | individual           1 1        
Issuance of common stock to nonemployees (in shares)           400,000 400,000        
Individual with stock grant agreement with issuance at IPO                      
Share-Based Compensation Awards                      
Issuance of common stock to nonemployees (in shares)   96,000