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Summary of significant accounting policies - Accounting for leases (Details)
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Property and equipment    
Leases, election of practical expedients package true  
Minimum    
Property and equipment    
Incremental borrowing rate 3.00% 3.00%
Maximum    
Property and equipment    
Incremental borrowing rate 4.30% 4.30%