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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK OPTIONS AND EMLOYEE STOCK PURCHASE PLAN (Details)
6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
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) 88.80% 87.40%
Expected volatility range, maximum (as a percent) 89.00% 88.40%
Risk-free interest rate range, minimum (as a percent) 1.21% 1.68%
Risk-free interest rate range, maximum (as a percent) 1.38% 1.70%
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)   72.10%
Expected volatility range, maximum (as a percent)   139.20%
Risk-free interest rate (as a percent) 0.28%  
Risk-free interest rate range, minimum (as a percent)   0.11%
Risk-free interest rate range, maximum (as a percent)   0.25%
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
Service-based restricted stock awards    
Stock-Based Compensation    
Vesting period of awards 4 years