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Stock Based Compensation (Details 4) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Employee stock options        
Weighted average assumptions for estimated fair value of employee stock options        
Expected term (years) 6 years 4 months 24 days 6 years 6 years 1 month 6 days 6 years
Expected volatility (as a percent) 86.50% 77.40% 76.60% 77.40%
Risk-free interest rate (as a percent) 1.95% 2.02% 1.62% 2.02%
Total unrecognized compensation expense, net of estimated forfeitures $ 11.3   $ 11.3  
Period to recognize total unrecognized compensation expense     3 years  
Employee Stock Purchase Plan        
Weighted average assumptions for estimated fair value of employee stock options        
Expected volatility (as a percent)   70.60%   70.60%
Employee Stock Purchase Plan | Minimum        
Weighted average assumptions for estimated fair value of employee stock options        
Expected term (years) 6 months 6 months 6 months 6 months
Expected volatility (as a percent) 70.60%   70.60%  
Risk-free interest rate (as a percent) 0.06% 0.06% 0.06% 0.06%
Employee Stock Purchase Plan | Maximum        
Weighted average assumptions for estimated fair value of employee stock options        
Expected term (years) 2 years 2 years 2 years 2 years
Expected volatility (as a percent) 82.90%   82.90%  
Risk-free interest rate (as a percent) 0.46% 0.46% 0.46% 0.46%