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, 2017
USD ($)
item
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
shares
Stock Based Compensation          
Number of homogenous groups for purposes of determining fair values of options | item 3        
Share-based compensation | $ $ 5,987,000 $ 7,832,000 $ 7,403,000    
Shares Available For Grant          
Outstanding at the end of the period (in shares) 11,696,696        
Additional disclosures          
Shares of common stock available for grant 11,696,696     11,696,696  
Shares reserved for future issuance       32,104,836  
Purchase plan          
Weighted-average assumptions relating to options granted          
Risk-free interest rate (as a percent) 0.50% 0.50% 0.60%    
Expected term (in years) 1 year 6 months 1 year 6 months 1 year 6 months    
Dividend yield (as a percent) 0.00% 0.00% 0.00%    
Expected volatility (as a percent) 63.10% 62.90% 61.20%    
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) 2.20% 1.80% 1.80%    
Expected term (in years) 6 years 7 months 6 days 6 years 2 months 12 days 6 years 6 months    
Dividend yield (as a percent) 0.00% 0.00% 0.00%    
Expected volatility (as a percent) 63.50% 61.10% 65.00%    
Shares Available For Grant          
Outstanding at the beginning of the period (in shares) 6,548,696        
Authorized for grant (in shares) 6,460,000        
Granted (in shares) (4,048,675) (5,251,185) (3,875,170)    
Cancelled (in shares) 2,736,675        
Outstanding at the end of the period (in shares) 11,696,696 6,548,696      
Number of Shares Underlying Options          
Outstanding at the beginning of the period (in shares) 20,257,233        
Granted (in shares) 4,048,675 5,251,185 3,875,170    
Exercised (in shares) (1,161,093)        
Cancelled (in shares) (2,736,675)        
Outstanding at the end of the period (in shares) 20,408,140 20,257,233      
Vested and expected to vest (in shares)       20,408,140  
Exercisable (in shares)       15,742,515  
Weighted Average Exercise Price          
Outstanding at the beginning of the period (in dollars per share) | $ / shares $ 6.24        
Granted (in dollars per share) | $ / shares 2.42        
Exercised (in dollars per share) | $ / shares 2.30        
Cancelled (in dollars per share) | $ / shares 8.18        
Outstanding at the end of the period (in dollars per share) | $ / shares $ 5.45 $ 6.24      
Vested and expected to vest (in dollars per share) | $ / shares       $ 5.45  
Exercisable (in dollars per share) | $ / shares       $ 6.25  
Weighted Average Remaining Contractual Term (in years)          
Outstanding at the end of the period 5 years 2 months 23 days        
Exercisable 4 years 1 month 28 days        
Aggregate Intrinsic Value          
Outstanding at the end of the period (in dollars) | $       $ 13,736,285  
Exercisable | $       $ 8,298,096  
Number of nonvested stock options (in shares)       4,665,624 3,668,891
Intrinsic value of nonvested stock options | $       $ 5,400,000 $ 31,000
Aggregate intrinsic value of options exercised | $ $ 1,200,000 $ 253,000      
Unrecognized compensation cost related to purchase plan | $       $ 91,000  
Weighted-average recognition period of unamortized compensation cost 2 years 3 months 18 days 6 months      
Number of shares vested 2,844,690 4,215,058      
Weighted-average exercise price for shares vested during the period (in dollars per share) | $ / shares $ 2.86 $ 2.70      
Additional disclosures          
Options cancelled (in shares) 2,736,675        
Options outstanding at the end of the period (in shares) 20,257,233 20,257,233   20,408,140 20,257,233
Grant-date weighted-average fair value (in dollars per share) | $ / shares $ 1.48 $ 1.72 $ 1.40    
Total unrecognized compensation costs | $       $ 6,200,000  
Shares of common stock available for grant 6,548,696 6,548,696   11,696,696 6,548,696
Options exercised during the period (in shares) 1,161,093        
Employee stock option | Consultant          
Stock Based Compensation          
Contractual term of the option 10 years        
Performance shares          
Stock Based Compensation          
Share-based compensation | $ $ 1,100,000 $ 1,100,000      
Shares Available For Grant          
Granted (in shares) (1,460,000)        
Number of Shares Underlying Options          
Granted (in shares) 1,460,000        
Additional disclosures          
Grant date fair value of performance stock options awards | $ $ 2,200,000 $ 1,100,000      
Total unrecognized compensation costs | $       $ 993,000  
Performance shares | Probable achievement of corresponding corporate-based milestones          
Shares Available For Grant          
Granted (in shares) (1,160,000) (700,000)      
Number of Shares Underlying Options          
Granted (in shares) 1,160,000 700,000