Shareholders' equity - Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended
	Oct. 29, 2016	Jul. 30, 2016	Apr. 30, 2016	Oct. 31, 2015	Aug. 01, 2015	May 02, 2015	Oct. 29, 2016	Oct. 31, 2015
Equity [Abstract]
Dividends declared per share (usd per share)	$ 0.26	$ 0.26	$ 0.26	$ 0.22	$ 0.22	$ 0.22	$ 0.78	$ 0.66
Dividends, common stock	$ 18.1	$ 19.7	$ 20.4	$ 17.5	$ 17.6	$ 17.6	$ 58.2	$ 52.7
Dividends on preferred shares	$ 18.1			$ 17.5			$ 18.1	$ 17.5