Annual report pursuant to Section 13 and 15(d)

STOCK-BASED COMPENSATION - Assumptions and Activity (Details)

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STOCK-BASED COMPENSATION - Assumptions and Activity (Details)
 12 Months Ended
Dec. 31, 2019
USD ($)
item
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
shares
Stock Based Compensation          
Number of homogenous groups for purposes of determining fair values of options | item 3        
Share-based compensation | $ $ 9,115,000 $ 7,704,000 $ 5,987,000    
Shares Available For Grant          
Outstanding at the end of the period (in shares) 16,615,971        
Additional disclosures          
Shares of common stock available for grant 16,615,971     16,615,971  
Shares reserved for future issuance       22,670,704  
Purchase Plan          
Weighted-average assumptions relating to options granted          
Risk-free interest rate (as a percent) 2.70% 2.40% 0.50%    
Expected term (in years) 1 year 6 months 1 year 3 months 18 days 1 year 6 months    
Dividend yield (as a percent) 0.00% 0.00% 0.00%    
Expected volatility (as a percent) 62.60% 66.20% 63.10%    
Employee stock option          
Stock Based Compensation          
Contractual term of the option 10 years        
Weighted-average assumptions relating to options granted          
Risk-free interest rate (as a percent) 2.40% 2.70% 2.20%    
Expected term (in years) 6 years 6 months 6 years 8 months 12 days 6 years 7 months 6 days    
Dividend yield (as a percent) 0.00% 0.00% 0.00%    
Expected volatility (as a percent) 65.50% 65.10% 63.50%    
Shares Available For Grant          
Outstanding at the beginning of the period (in shares) 15,097,014        
Authorized for grant (in shares) 3,544,984        
Granted (in shares) (7,457,575) (4,594,225) (4,048,675)    
Cancelled (in shares) 5,431,548        
Outstanding at the end of the period (in shares) 16,615,971 15,097,014      
Number of Shares Underlying Options          
Outstanding at the beginning of the period (in shares) 20,713,331        
Granted (in shares) 7,457,575 4,594,225 4,048,675    
Exercised (in shares) (68,654)        
Cancelled (in shares) (5,431,548)        
Outstanding at the end of the period (in shares) 22,670,704 20,713,331      
Vested and expected to vest (in shares)       22,363,204  
Exercisable (in shares)       15,804,733  
Weighted Average Exercise Price          
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 4.20        
Granted (in dollars per share) | $ / shares 2.03        
Exercised (in dollars per share) | $ / shares 2.05        
Cancelled (in dollars per share) | $ / shares 4.16        
Outstanding at the end of the period (in dollars per share) | $ / shares $ 3.51 $ 4.20      
Vested and expected to vest (in dollars per share) | $ / shares       $ 3.51  
Exercisable (in dollars per share) | $ / shares       $ 3.97  
Weighted Average Remaining Contractual Term (in years)          
Outstanding at the end of the period 6 years 1 month 24 days        
Exercisable 5 years 1 month 6 days        
Aggregate Intrinsic Value          
Outstanding at the end of the period (in dollars) | $       $ 809,459,000  
Exercisable | $       $ 243,612,000  
Number of nonvested stock options (in shares)       6,865,971 5,962,769
Intrinsic value of nonvested stock options | $       $ 614,000 $ 121,000
Aggregate intrinsic value of options exercised | $ $ 12,000 $ 1,300,000      
Weighted-average recognition period of unamortized compensation cost 2 years 3 months 18 days 4 months 24 days      
Number of shares vested 4,442,936 2,924,823      
Weighted-average exercise price for shares vested during the period (in dollars per share) | $ / shares $ 3.26 $ 2.88      
Additional disclosures          
Options cancelled (in shares) 5,431,548        
Options outstanding at the end of the period (in shares) 22,670,704 20,713,331   22,670,704 20,713,331
Grant-date weighted-average fair value (in dollars per share) | $ / shares $ 1.27 $ 2.66 $ 1.48    
Total unrecognized compensation costs | $       $ 8,900,000  
Shares of common stock available for grant 16,615,971 15,097,014   16,615,971 15,097,014
Options exercised during the period (in shares) 68,654        
Employee stock option | Consultant          
Stock Based Compensation          
Contractual term of the option 10 years        
Employee stock option | Purchase Plan          
Aggregate Intrinsic Value          
Unrecognized compensation cost related to purchase plan | $       $ 475,000,000,000  
Performance shares          
Shares Available For Grant          
Granted (in shares) (307,500)        
Number of Shares Underlying Options          
Granted (in shares) 307,500        
Performance shares | Vesting upon achievement of corporate performance-based milestones          
Additional disclosures          
Total unrecognized compensation costs | $       $ 363,000