Stock-Based Compensation (Details 2) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Summary of stock-based compensation expense      
Share-based compensation expense included in operating expenses $ 7,070,000 $ 1,508,000 $ 1,795,000
Total unrecognized compensation expense 16,000,000    
Weighted average period over which unrecognized compensation is expected to be recognized 3 years    
Research and development      
Summary of stock-based compensation expense      
Share-based compensation expense included in operating expenses 1,953,000 302,000 338,000
General and administrative      
Summary of stock-based compensation expense      
Share-based compensation expense included in operating expenses $ 5,117,000 $ 1,206,000 $ 1,457,000