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STOCKHOLDERS' EQUITY - Narrative (Details) - USD ($)
 1 Months Ended 12 Months Ended
May 31, 2022
Dec. 31, 2022
May 31, 2021
Aug. 31, 2020
Apr. 30, 2020
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Jun. 30, 2023
Jun. 30, 2020
Jun. 29, 2020
May 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Preferred stock, shares authorized (in shares)   5,000,000       5,000,000 5,000,000          
Preferred stock, shares issued (in shares)   0       0 0          
Preferred stock, shares outstanding (in shares)   0       0 0          
Common stock, shares authorized (in shares)   640,000,000       640,000,000 640,000,000   640,000,000 320,000,000 160,000,000  
Common stock, shares outstanding (in shares)   166,793,320       178,133,548 166,793,320          
Stock option maximum term           4 years 7 months 9 days            
Specified stockholder ownership percentage 10.00%                      
Purchase price of common stock percent under stock incentive plans for specified stockholder 110.00%                      
Stock option maximum term for specified stockholder 5 years                      
Share of common stock subject to a stock option or stock appreciation right (in shares) 100.00%                      
Shares reserved for issuance of future awards (in shares)           2,858,653            
Intrinsic value of options exercised           $ 0 $ 0 $ 2,800,000        
Restricted stock units awarded (in shares)           4,811,834            
Total payments for the employees' tax obligations to taxing authorities           $ 1,453,000 $ 2,104,000 $ 3,258,000        
Restricted Stock Units (RSUs)                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock options vesting period           3 years            
Restricted stock units awarded (in shares)           4,811,834 4,349,795 2,140,785        
Grant date fair value per award of restricted stock units (in dollars per share)           $ 2.29 $ 5.52 $ 11.16        
Aggregate fair value of RSUs vested in period           $ 17,800,000 $ 13,100,000 $ 9,000,000        
Shares withheld from issuance in order to pay employee taxes (in shares)           788,236 380,917 293,120        
Restated 2018 Stock Incentive Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Increase in capital shares reserved for future issuance (in shares) 7,900,000               10,000,000     9,900,000
2018 Stock Incentive Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Percent of fair value per share of common stock on option grant date 100.00%                      
Stock options expiration term 10 years                      
Fixed ratio shares of common stock 1.33                      
Shares reserved for issuance of future awards (in shares)           13,738,867            
2010 Employee Stock Purchase Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Percent of fair value per share of common stock on option grant date     85.00%                  
Stock reserved for issuance under plan (in shares)     5,000,000                  
Purchase plan offering period     2 years                  
Purchase plan purchase period     6 months                  
Maximum | 2018 Stock Incentive Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock option maximum term 10 years                      
Stock options vesting period 4 years                      
Minimum | 2018 Stock Incentive Plan                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock options vesting period 3 years                      
Jefferies LLC | ATM Agreement                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Stock offering program, maximum value       $ 150,000,000                
Stock offering program, increase to aggregate offering price   $ 175,000,000                    
Amount remained available           $ 194,500,000            
Issuance of common stock in at-the-market offering, net of offering expenses (in shares)           8,249,261 19,300,743          
Consideration received from sale of stock           $ 15,100,000 $ 84,900,000          
Stock Purchase Agreement | Biogen MA, Inc.                        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Consideration received from sale of stock         $ 225,000,000