Shareholders' equity - Additional Information (Detail) - 2016 Program - USD ($)	1 Months Ended
	Aug. 31, 2016	Apr. 29, 2017	Feb. 29, 2016
Class of Stock [Line Items]
Amount authorized	$ 1,375,000,000	$ 1,375,000,000	$ 750,000,000
Increase in authorized share repurchase program amount	$ 625,000,000