Shareholders' equity - Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jul. 01, 2018	Jun. 30, 2019	Jul. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 279,583	$ 265,088	$ 548,425	$ 521,048
Total before tax	(87,172)	(7,024)	(140,062)	(68,197)
Taxes on income from continuing operations	(3,844)	(9,576)	(14,816)	(15,818)
Net of tax	(83,328)	2,552	(125,246)	(52,379)
Interest expense	20,758	26,649	43,450	52,592
Total reclassifications, net of tax	(2,564)	1,258	(4,329)	1,897
Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,738	1,734	3,478	3,480
Prior-service costs
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	22	23	44	47
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	1,760	1,757	3,522	3,527
Tax benefit	(410)	(408)	(821)	(819)
Total reclassifications, net of tax	1,350	1,349	2,701	2,708
Reclassification out of Accumulated Other Comprehensive Income | Foreign exchange contract | Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	(179)	(118)	(365)	(951)
Total before tax	(179)	(118)	(365)	(951)
Taxes on income from continuing operations	(71)	(27)	(94)	(140)
Net of tax	(108)	(91)	(271)	(811)
Reclassification out of Accumulated Other Comprehensive Income | Cross-currency interest rate swap | Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	4,917	0	8,799	0
Taxes on income from continuing operations	1,111	0	2,040	0
Net of tax	(3,806)	0	(6,759)	0
Interest expense	$ (4,917)	$ 0	$ (8,799)	$ 0