Redeemable preferred shares - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Oct. 05, 2016	Aug. 04, 2018	Jul. 29, 2017	Aug. 04, 2018	Jul. 29, 2017	Feb. 03, 2018
Temporary Equity [Line Items]
Accretion on redeemable convertible preferred shares				$ 16.4
Series A Redeemable Convertible Preferred Stock
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
Preferred stock, dividend rate, percentage				5.00%
Payments of stock issuance costs	$ 13.7
Accumulated accretion of dividends		$ 3.1	$ 1.4	$ 3.1	$ 1.4	$ 2.3
Accretion on redeemable convertible preferred shares		$ 0.4	$ 0.4	$ 0.8	$ 0.8