Recently issued accounting standards Recently issued accounting standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Oct. 01, 2017	Jul. 02, 2017	Apr. 02, 2017	Dec. 31, 2016	Sep. 25, 2016	Jun. 26, 2016	Mar. 27, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Net income (loss) attributable to parent	$ (42,658)	$ 77,015	$ 78,003	$ 40,170	$ 61,063	$ 66,322	$ 59,303	$ 50,689	$ 152,530	$ 237,377	$ 244,863
Net income (loss), diluted (in dollar per share)	$ (0.91)	$ 1.65	$ 1.67	$ 0.86	$ 1.30	$ 1.40	$ 1.26	$ 1.04	$ 3.27	$ 4.98	$ 5.10
Accounting Standards Update 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Net income (loss) attributable to parent									$ 6,600
Net income (loss), diluted (in dollar per share)									$ 0.14