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Related party transactions (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Collaboration arrangement    
Related Party Transaction [Line Items]    
Revenue recognized   $ 4,800,000
Investor | Consulting and management services    
Related Party Transaction [Line Items]    
Total amount of services received $ 10,000 3,000
Genzyme | Collaboration arrangement    
Related Party Transaction [Line Items]    
Revenue recognized 1,500,000  
Expense recognized $ 43,800 $ 500,000