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Summary of significant accounting policies - Liability for Sale of Future Royalties (Details) - USD ($)
$ in Thousands
 9 Months Ended
Jul. 17, 2020
Sep. 30, 2020
Sep. 30, 2019
Assigned royalty payments      
Cash consideration received from Royalty Purchase Agreement $ 650,000 $ 650,000 $ 0
Assigned royalty payment, retained percentage 57.067%    
Revenue recognition, milestone, potential achievements $ 362,500    
Royalty Purchase Agreement | Assigned Royalty Payments      
Assigned royalty payments      
Assigned royalty payment, percentage 42.933%    
Cash consideration received from Royalty Purchase Agreement $ 650,000    
Royalty purchase agreement, termination period 60 days    
Royalty purchase agreement, payment maximum $ 1,300,000