Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Apr. 01, 2018	Apr. 02, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 255,960	$ 232,321
Total before tax	(61,173)	(37,680)
Taxes (benefit) on income from continuing operations	6,242	(2,669)
Net of tax	(54,931)	(40,349)
Total reclassifications, net of tax	639	2,509
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,746	1,726
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	24	29
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,770	1,755
Tax benefit	(411)	(624)
Total reclassifications, net of tax	1,359	1,131
Reclassification out of Accumulated Other Comprehensive Income [Member] | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(833)	1,645
Total before tax	(833)	1,645
Taxes (benefit) on income from continuing operations	113	(267)
Net of tax	$ (720)	$ 1,378