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Redeemable preferred shares - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 6 Months Ended
Oct. 05, 2016
Aug. 03, 2019
May 04, 2019
Aug. 04, 2018
May 05, 2018
Aug. 03, 2019
Aug. 04, 2018
Feb. 02, 2019
Temporary Equity [Line Items]                
Accretion on redeemable convertible preferred shares   $ 8.2 $ 8.2 $ 8.2 $ 8.2      
Series A Redeemable Convertible Preferred Stock                
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   4.8   3.1   $ 4.8 $ 3.1 $ 4.0
Accretion on redeemable convertible preferred shares   $ 0.4   $ 0.4   $ 0.8 $ 0.8