New accounting standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Oct. 01, 2017	Sep. 25, 2016	Oct. 01, 2017	Sep. 25, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income attributable to common shareholders	$ 77,015	$ 66,322	$ 195,188	$ 176,314
Net income (in dollars per share)	$ 1.65	$ 1.40	$ 4.18	$ 3.69
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net income attributable to common shareholders	$ 1,500		$ 5,900
Net income (in dollars per share)	$ 0.03		$ 0.13