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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMPLOYEE STOCK PURCHASE PLAN (Details)
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
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, minimum (as a percent) 88.40% 88.80%
Expected volatility range, maximum (as a percent) 89.20% 89.00%
Risk-free interest rate range, minimum (as a percent) 1.98% 1.21%
Risk-free interest rate range, maximum (as a percent) 1.99% 1.38%
Expected term 5 years 6 months 5 years 6 months
Employee Stock Purchase Plan    
Assumptions used to estimate fair value of awards    
Dividend yield (as a percent) 0.00% 0.00%
Expected volatility (as a percent)   68.40%
Expected volatility range, minimum (as a percent) 57.70%  
Expected volatility range, maximum (as a percent) 64.10%  
Risk-free interest rate (as a percent)   0.28%
Risk-free interest rate range, minimum (as a percent) 0.45%  
Risk-free interest rate range, maximum (as a percent) 0.89%  
Expected term   12 months
Employee Stock Purchase Plan | Minimum    
Assumptions used to estimate fair value of awards    
Expected term 6 months  
Employee Stock Purchase Plan | Maximum    
Assumptions used to estimate fair value of awards    
Expected term 12 months