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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Stock Options    
Stock-Based Compensation    
Vesting period of awards 4 years  
Assumptions used to estimate fair value of awards    
Dividend yield (as a percent) 0.00% 0.00%
Expected volatility range, (as a percent)   82.10%
Expected volatility range, minimum (as a percent) 92.50%  
Expected volatility range, maximum (as a percent) 98.00%  
Risk-free interest rate range, (as a percent)   2.55%
Risk-free interest rate range, minimum (as a percent) 2.24%  
Risk-free interest rate range, maximum (as a percent) 2.56%  
Expected term range   5 years 3 months
Stock Options | Minimum    
Assumptions used to estimate fair value of awards    
Expected term range 5 years 3 months  
Stock Options | Maximum    
Assumptions used to estimate fair value of awards    
Expected term range 6 years 5 months 8 days  
Employee Stock Purchase Plan    
Assumptions used to estimate fair value of awards    
Dividend yield (as a percent) 0.00% 0.00%
Expected volatility range, minimum (as a percent) 133.30% 43.70%
Expected volatility range, maximum (as a percent) 165.30% 47.50%
Risk-free interest rate range, minimum (as a percent) 2.56% 1.53%
Risk-free interest rate range, maximum (as a percent) 2.63% 1.76%
Employee Stock Purchase Plan | Minimum    
Assumptions used to estimate fair value of awards    
Expected term range 6 months 6 months
Employee Stock Purchase Plan | Maximum    
Assumptions used to estimate fair value of awards    
Expected term range 12 months 12 months