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Related party transactions (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Investor | Consulting and management services    
Related Party Transaction [Line Items]    
Total amount of services received $ 0 $ 10,000
Genzyme | Collaboration arrangement    
Related Party Transaction [Line Items]    
Revenue recognized 500,000 1,500,000
Expense recognized $ 200,000 $ 43,800