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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details 4) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Stock-Based Compensation        
Stock-based compensation expense included in operating expenses $ 2,009 $ 1,222 $ 5,752 $ 3,456
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)     83.50% 50.60%
Expected volatility range, maximum (as a percent)     166.60% 139.10%
Risk-free interest rate range, minimum (as a percent)     0.06% 0.09%
Risk-free interest rate range, maximum (as a percent)     0.15% 0.21%
Employee Stock Purchase Plan | Minimum
        
Assumptions used to estimate fair value of awards        
Expected term     6 months 6 months
Employee Stock Purchase Plan | Maximum
        
Assumptions used to estimate fair value of awards        
Expected term     12 months 12 months
Service-based restricted stock awards
        
Stock-Based Compensation        
Vesting period of awards     4 years  
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)     89.80% 74.20%
Expected volatility range, maximum (as a percent)     92.20% 75.90%
Risk-free interest rate range, minimum (as a percent)     1.64% 0.80%
Risk-free interest rate range, maximum (as a percent)     1.92% 1.81%
Expected term     5 years 6 months 6 years