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Redeemable preferred shares - Additional Information (Details) - Series A Redeemable Convertible Preferred Stock - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended
Oct. 05, 2016
Aug. 01, 2020
May 02, 2020
May 04, 2019
Feb. 01, 2020
Temporary Equity [Line Items]          
Redeemable convertible preferred stock, shares issued (shares) 625,000        
Preferred stock, purchase price $ 625.0        
Shares issued, price per share (in usd per share) $ 1,000        
Preferred stock, dividend rate, percentage     5.00%    
Payments of stock issuance costs $ 13.7        
Accumulated accretion of dividends     $ 6.1 $ 4.4 $ 5.7
Accretion on redeemable convertible preferred shares     $ 0.4 $ 0.4  
Scenario, Forecast          
Temporary Equity [Line Items]          
Preferred stock, increase in stated value (in usd per share)   $ 12.50