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Stock-based Compensation (Details 4)
9 Months Ended 12 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Dec. 31, 2014
Dec. 31, 2013
Weighted-average assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants        
Risk-free interest rate 1.70% 1.80% 1.80%  
Volatility 77.00% 77.00% 77.10% 87.80%
Expected term (in years) 6 years 2 months 16 days 6 years 3 months 6 years 3 months 6 years 3 months