Shareholders' equity - Share Repurchase (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	May 05, 2018	Apr. 29, 2017
Class of Stock [Line Items]
Shares repurchased (shares)	1.5	0.0
Amount repurchased	$ 60.0	$ 0.0
Average repurchase price per share (usd per share)	$ 39.62	$ 0.00
2017 Program
Class of Stock [Line Items]
Amount authorized	$ 600.0
Shares repurchased (shares)	0.2
Amount repurchased	$ 9.4
Average repurchase price per share (usd per share)	$ 38.86
2016 Program
Class of Stock [Line Items]
Amount authorized	$ 1,375.0
Shares repurchased (shares)	1.3	0.0
Amount repurchased	$ 50.6	$ 0.0
Average repurchase price per share (usd per share)	$ 39.76	$ 0.00
Remaining authorized repurchase amount	$ 590.6