Redeemable preferred shares - Additional Information (Details) - Series A Redeemable Convertible Preferred Stock - USD ($) $ / shares in Units, $ in Millions		3 Months Ended			6 Months Ended
	Oct. 05, 2016	Oct. 30, 2020	Aug. 01, 2020	Aug. 03, 2019	Aug. 01, 2020	Aug. 03, 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%
Preferred stock, increase in stated value (in usd per share)			$ 12.50
Payments of stock issuance costs	$ 13.7
Accumulated accretion of dividends			$ 6.5	$ 4.8	$ 6.5	$ 4.8	$ 5.7
Accretion on redeemable convertible preferred shares			$ 0.4	$ 0.4	$ 0.8	$ 0.8
Scenario, Forecast
Temporary Equity [Line Items]
Preferred stock, increase in stated value (in usd per share)		$ 12.66