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Stockholders' equity - Equity offerings (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Oct. 31, 2020
Jun. 30, 2020
Dec. 31, 2019
Nov. 30, 2019
Mar. 31, 2021
Mar. 31, 2020
Class of Warrant or Right [Line Items]            
Net proceeds from issuance after deducting underwriting discounts, commissions and other offering expenses $ 269,975 $ 107,782 $ 85,598      
Underwriting discounts, commissions and other offering expenses         $ 100 $ 355
Number of common stocks are exercisable from the November 2019 Pre-Funded Warrants and the June 2020 Pre-Funded Warrants         5,284,238  
Pre-Funded Warrants            
Class of Warrant or Right [Line Items]            
Issuance of pre-funded warrants (in shares) 1,562,500 1,521,738        
Public offering price $ 39.9999 $ 22.9999   $ 13.6099    
Variation between public offering prices of common stock and Pre-Funded Warrants $ 0.0001 $ 0.0001   $ 0.0001    
Maximum percentage of holding for exercise warrants 9.99% 9.99%   9.99%    
Prior notice period to change percentage of holding 61 days 61 days        
Maximum percentage of increase (decrease) of holding for exercise warrants 19.99% 19.99%        
Underwriter rights            
Class of Warrant or Right [Line Items]            
Number of shares issued 5,625,000 3,478,261 838,530 4,516,561    
Issuance of pre-funded warrants (in shares)       2,200,000    
Public offering price $ 40.00 $ 23.00   $ 13.61    
Shares issued during period 937,500 652,173        
Underwriting discounts, commissions and other offering expenses $ 17,525          
November 2019 Underwriting Agreement            
Class of Warrant or Right [Line Items]            
Underwriting discounts, commissions and other offering expenses     $ 5,814      
June 2020 Underwriting Agreement            
Class of Warrant or Right [Line Items]            
Underwriting discounts, commissions and other offering expenses   $ 7,217