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Summary of significant accounting policies - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Lessee, Lease, Description [Line Items]          
Warrants, notice period for nominal cash consideration     61 days    
Expected dividends 0.00% 0.00% 0.00% 0.00%  
Prefunded Warrants          
Lessee, Lease, Description [Line Items]          
Outstanding pre-funded warrants (in shares) 12,663,010 12,663,010 12,663,010 12,663,010  
Warrants, notice period for nominal cash consideration     61 days 61 days  
Money market funds          
Lessee, Lease, Description [Line Items]          
Money market funds $ 152.4   $ 152.4   $ 108.3
Maximum | Office space          
Lessee, Lease, Description [Line Items]          
Lease term 12 months   12 months