Shareholders' equity - Dividends (Details) - USD ($)	3 Months Ended
	May 05, 2018	Apr. 29, 2017	Apr. 30, 2016
Dividends Payable [Line Items]
Dividends declared per share (usd per share)	$ 0.37	$ 0.31000000
Dividends, common stock	$ 21,800,000	$ 21,300,000
Cash dividends declared	0
Dividends on redeemable convertible preferred shares	8,200,000
Series A Redeemable Convertible Preferred Stock
Dividends Payable [Line Items]
Cash dividends declared		7,800,000	$ 7,800,000
Dividends on redeemable convertible preferred shares	400,000	400,000
Accrued Expenses
Dividends Payable [Line Items]
Cash dividends declared	$ 7,800,000
Other Liabilities
Dividends Payable [Line Items]
Cash dividends declared		$ 7,800,000