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Redeemable preferred hares - Narrative (Details) - Series A Redeemable Convertible Preferred Stock - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Oct. 05, 2018
Oct. 05, 2016
Feb. 02, 2019
Feb. 03, 2018
Temporary Equity [Line Items]        
Redeemable convertible preferred stock, shares issued   625,000    
Preferred stock, purchase price   $ 625.0    
Shares issued, price per share   $ 1,000    
Payments of stock issuance costs   $ 13.7    
Accumulated accretion of dividends     $ 4.0 $ 2.3
Preferred stock, dividend rate, percentage     5.00%  
Percentage exceeding applicable conversion price 175.00%      
Threshold period at which shares can be converted 20 days      
Percentage of cash equal to the stated value     101.00%  
Maximum        
Temporary Equity [Line Items]        
Preferred dividends, percentage of average quarterly cash dividends (not more than)     130.00%