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STOCKHOLDERS' EQUITY - EQUITY PLANS - SCHEDULE OF AGGREGATE OPTION AND AWARD ACTIVITY (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Shares Available For Grant      
Balance at the beginning of the period (in shares) 6,407,355    
Additional shares authorized (in shares) 7,000,000    
Options granted (in shares) (6,531,931)    
Awards granted (in shares) (17,986)    
Options cancelled/forfeited/expired (in shares) 400,309    
Balance at the end of the period (in shares) 7,257,747 6,407,355  
Number of Shares - Options Outstanding      
Balance at the beginning of the period (in shares) 37,614,013    
Options granted (in shares) 6,531,931    
Options exercised (in shares) (21,944)    
Options cancelled/forfeited/expired (in shares) (460,309)    
Balance at the end of the period (in shares) 43,663,691 37,614,013  
Options exercisable at the end of the period (in shares) 25,721,508 19,915,713 16,464,746
Options fully vested and expected to vest at the end of the period (in shares) 42,621,956    
Weighted Average Exercise Price Per Share - Options Outstanding      
Balance at the beginning of the period (in dollars per share) $ 2.26    
Options granted (in dollars per share) 1.37    
Options exercised (in dollars per share) 1.16    
Options cancelled/forfeited/expired (in dollars per share) 3.48    
Balance at the end of the period (in dollars per share) 2.12 $ 2.26  
Options exercisable at the end of the period (in dollars per share) 2.54 $ 2.86 $ 3.13
Options fully vested and expected to vest at the end of the period (in dollars per share) $ 2.13    
Weight Average Remaining Contractual Life (in years) - Options Outstanding      
Balance at the end of the period 6 years 3 months 25 days    
Options exercisable at the end of the period 4 years 10 months 24 days    
Options fully vested and expected to vest at the end of the period 6 years 3 months 7 days    
Aggregate Intrinsic Value - Options Outstanding      
Balance at the end of the period (in dollars) $ 5,624    
Options exercisable at the end of the period (in dollars) 2,247    
Options fully vested and expected to vest at the end of the period (in dollars) $ 5,437