Income Tax Rate reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Income Tax [Abstract]
Expected federal income tax expense at statutory rate	$ 13,467	$ 17,861	$ 28,400	$ 32,430
State income taxes before valuation allowance, net of federal tax effect	203	159	698	209
Effect of foreign source income	(4,142)	(7,273)	(2,346)	(10,645)
Tax contingencies	45	406	813	535
Valuation allowance	(188)	756	453	1,237
U.S. federal research credit	(674)	(549)	(1,574)	(1,148)
Equity compensation	(228)	(497)	(5,792)	(2,792)
Global Intangible Low Tax Income	835	0	1,949	0
Other items, net	464	179	727	327
Income tax expense	$ 9,782	$ 11,042	$ 23,328	$ 20,153