Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Apr. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 268,842	$ 255,960
Total before tax	(52,890)	(61,173)
Taxes on income from continuing operations	(10,972)	(6,242)
Net of tax	(41,918)	(54,931)
Interest expense	22,692	25,943
Total reclassifications, net of tax	(1,765)	639
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,740	1,746
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	22	24
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,762	1,770
Tax benefit	(411)	(411)
Total reclassifications, net of tax	1,351	1,359
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(186)	(833)
Total before tax	(186)	(833)
Taxes on income from continuing operations	(23)	(113)
Net of tax	(163)	(720)
Reclassification out of Accumulated Other Comprehensive Income | Cross-currency interest rate swap | Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	3,882	0
Taxes on income from continuing operations	929	0
Net of tax	2,953	0
Interest expense	$ (3,882)	$ 0