Inventories (Details) - USD ($) $ in Thousands	Apr. 01, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Raw materials	$ 170,814	$ 172,960
Finished goods	78,908	68,851
Provision for obsolete or aged inventory	(8,308)	(7,769)
Inventories, net	$ 241,414	$ 234,042