Redeemable preferred shares - Additional Information (Details) $ / shares in Units, $ in Millions		3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 05, 2016 USD ($) $ / shares shares	Jul. 29, 2017 USD ($) shares	Jul. 29, 2017 USD ($) $ / shares shares	Jan. 28, 2017 USD ($) $ / shares shares
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 | shares	625,000
Preferred stock, purchase price	$ 625.0
Shares issued, price per share | $ / shares	$ 1,000
Payments of stock issuance costs	$ 13.7
Accumulated accretion of dividends		$ 1.4	1.4	$ 0.6
Accretion on redeemable convertible preferred shares		$ 0.4	$ 0.8
Conversion ratio			10.7707	10.6529
Conversion price (USD per share) | $ / shares			$ 92.8445	$ 93.8712
Conversion of stock, shares issued | shares		6,700,000	6,700,000	6,700,000
Preferred stock, dividend rate, percentage			5.00%
Liquidation preference		$ 632.8	$ 632.8