Shareholders' equity - Share Repurchase (Details) - USD ($) $ / shares in Units, shares in Millions	6 Months Ended
	Jul. 30, 2016	Aug. 01, 2015	Feb. 29, 2016	Jun. 14, 2013
Class of Stock [Line Items]
Shares repurchased (shares)	3.9	0.6
Amount repurchased	$ 375,000,000	$ 81,900,000
Average repurchase price per share (usd per share)	$ 95.49	$ 130.27
2013 Program
Class of Stock [Line Items]
Amount authorized				$ 350,000,000
Shares repurchased (shares)	1.2	0.6
Amount repurchased	$ 135,600,000	$ 81,900,000
Average repurchase price per share (usd per share)	$ 111.26	$ 130.27
2016 Program
Class of Stock [Line Items]
Amount authorized			$ 750,000,000
Shares repurchased (shares)	2.7
Amount repurchased	$ 239,400,000
Average repurchase price per share (usd per share)	$ 88.39