Restructuring, acquisition and integration-related costs - Restructuring and Related Costs by Type of Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2017	Apr. 02, 2016
Restructuring Cost and Reserve [Line Items]
Restructuring, acquisition and integration-related costs	$ 3,025	$ 5,253
Cost of sales
Restructuring Cost and Reserve [Line Items]
Restructuring costs	2,898	5,026
Acquisition integration-related costs	(86)	822
Restructuring, acquisition and integration-related costs	2,812	5,848
Selling, general and administrative expenses
Restructuring Cost and Reserve [Line Items]
Restructuring costs	127	227
Acquisition integration-related costs	1,039	967
Restructuring, acquisition and integration-related costs	$ 1,166	$ 1,194