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Stock option plan (Details 4) (USD $)
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Share-based compensation expense recorded in the statement of operations      
Share-based compensation expense $ 8,427,188 $ 2,288,392 $ 2,816,637
Unrecognized compensation cost 21,600,000    
Weighted average remaining service period for recognition of unrecognized compensation cost 2 years 5 months 26 days    
Research and development
     
Share-based compensation expense recorded in the statement of operations      
Share-based compensation expense 2,039,880 804,576 916,495
General and administrative
     
Share-based compensation expense recorded in the statement of operations      
Share-based compensation expense $ 6,387,308 $ 1,483,816 $ 1,900,142