Shareholders' equity - Additional Information (Detail) - USD ($)	1 Months Ended
	Aug. 31, 2016	Oct. 28, 2017	Jun. 30, 2017	Feb. 29, 2016
2016 Program
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
2017 Program
Class of Stock [Line Items]
Amount authorized		$ 650,600,000	$ 600,000,000