Shareholders' equity - Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	May 04, 2019	May 05, 2018
Dividends Payable [Line Items]
Dividends declared per common share (usd per share)	$ 0.37	$ 0.37
Dividends, common stock	$ 19.3	$ 21.8
Dividends declared per preferred share (usd per share)	$ 12.50	$ 12.50
Dividends on redeemable convertible preferred shares	$ 8.2	$ 8.2
Series A Redeemable Convertible Preferred Stock
Dividends Payable [Line Items]
Dividends declared per preferred share (usd per share)	$ 12.5	$ 12.5
Dividends, preferred stock	$ 7.8	$ 7.8
Dividends on redeemable convertible preferred shares	0.4	0.4
Accrued Expenses
Dividends Payable [Line Items]
Dividends, preferred stock	$ 7.8
Other liabilities
Dividends Payable [Line Items]
Dividends, preferred stock		$ 7.8