Annual report pursuant to Section 13 and 15(d)

STOCK-BASED COMPENSATION - Assumptions (Details)

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STOCK-BASED COMPENSATION - Assumptions (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
item
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
shares
Stock Based Compensation          
Number of homogenous groups for purposes of determining fair values of options | item 3        
Share-based compensation | $ $ 7,704,000 $ 5,987,000 $ 7,832,000    
Shares Available For Grant          
Outstanding at the end of the period (in shares) 15,097,014        
Additional disclosures          
Shares of common stock available for grant 15,097,014     15,097,014  
Shares reserved for future issuance       20,713,331  
Purchase Plan          
Weighted-average assumptions relating to options granted          
Risk-free interest rate (as a percent) 2.40% 0.50% 0.50%    
Expected term (in years) 1 year 3 months 18 days 1 year 6 months 1 year 6 months    
Dividend yield (as a percent) 0.00% 0.00% 0.00%    
Expected volatility (as a percent) 66.20% 63.10% 62.90%    
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) 2.70% 2.20% 1.80%    
Expected term (in years) 6 years 8 months 12 days 6 years 7 months 6 days 6 years 2 months 12 days    
Dividend yield (as a percent) 0.00% 0.00% 0.00%    
Expected volatility (as a percent) 65.10% 63.50% 61.10%    
Shares Available For Grant          
Outstanding at the beginning of the period (in shares) 11,696,696        
Authorized for grant (in shares) 4,878,124        
Granted (in shares) (4,594,225) (4,048,675) (5,251,185)    
Cancelled (in shares) 3,116,419        
Outstanding at the end of the period (in shares) 15,097,014 11,696,696      
Number of Shares Underlying Options          
Outstanding at the beginning of the period (in shares) 20,408,140        
Granted (in shares) 4,594,225 4,048,675 5,251,185    
Exercised (in shares) (1,172,615)        
Cancelled (in shares) (3,116,419)        
Outstanding at the end of the period (in shares) 20,713,331 20,408,140      
Vested and expected to vest (in shares)       20,513,331  
Exercisable (in shares)       14,750,561  
Weighted Average Exercise Price          
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 5.45        
Granted (in dollars per share) | $ / shares 4.19        
Exercised (in dollars per share) | $ / shares 2.75        
Cancelled (in dollars per share) | $ / shares 12.87        
Outstanding at the end of the period (in dollars per share) | $ / shares $ 4.20 $ 5.45      
Vested and expected to vest (in dollars per share) | $ / shares       $ 4.21  
Exercisable (in dollars per share) | $ / shares       $ 4.39  
Weighted Average Remaining Contractual Term (in years)          
Outstanding at the end of the period 5 years 11 months 16 days        
Exercisable 4 years 10 months 2 days        
Aggregate Intrinsic Value          
Outstanding at the end of the period (in dollars) | $       $ 701,842  
Exercisable | $       $ 580,787  
Number of nonvested stock options (in shares)       5,962,769 4,665,624
Intrinsic value of nonvested stock options | $       $ 121,000 $ 5,400,000
Aggregate intrinsic value of options exercised | $ $ 1,300,000 $ 1,200,000      
Weighted-average recognition period of unamortized compensation cost 2 years 7 months 6 days 9 months 18 days      
Number of shares vested 2,924,823 2,844,690      
Weighted-average exercise price for shares vested during the period (in dollars per share) | $ / shares $ 2.88 $ 2.86      
Additional disclosures          
Options cancelled (in shares) 3,116,419        
Options outstanding at the end of the period (in shares) 20,408,140 20,408,140   20,713,331 20,408,140
Grant-date weighted-average fair value (in dollars per share) | $ / shares $ 2.66 $ 1.48 $ 1.72    
Total unrecognized compensation costs | $       $ 10,900,000  
Shares of common stock available for grant 11,696,696 11,696,696   15,097,014 11,696,696
Options exercised during the period (in shares) 1,172,615        
Employee stock option | Purchase Plan          
Aggregate Intrinsic Value          
Unrecognized compensation cost related to purchase plan | $       $ 1,100,000  
Performance shares          
Shares Available For Grant          
Granted (in shares) (200,000)        
Number of Shares Underlying Options          
Granted (in shares) 200,000        
Performance shares | Vesting upon achievement of corporate performance-based milestones          
Additional disclosures          
Total unrecognized compensation costs | $       $ 385,000