Shareholders' equity - Share Repurchase (Details) - USD ($) $ / shares in Units, shares in Millions	6 Months Ended
	Jul. 29, 2017	Jul. 30, 2016	Aug. 31, 2016	Feb. 29, 2016
Class of Stock [Line Items]
Shares repurchased (shares)	8.1	3.9
Amount repurchased	$ 460,000,000	$ 375,000,000
Average repurchase price per share (usd per share)	$ 56.91	$ 95.49
2016 Program
Class of Stock [Line Items]
Amount authorized	$ 1,375,000,000		$ 1,375,000,000	$ 750,000,000
Shares repurchased (shares)	8.1	2.7
Amount repurchased	$ 460,000,000	$ 239,400,000
Average repurchase price per share (usd per share)	$ 56.91	$ 88.39
Remaining authorized repurchase amount	$ 50,600,000
2013 Program
Class of Stock [Line Items]
Amount authorized		$ 350,000,000
Shares repurchased (shares)		1.2
Amount repurchased		$ 135,600,000
Average repurchase price per share (usd per share)		$ 111.26