Shareholders' equity - Change in accumulated other comprehensive income, net of tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jul. 01, 2018	Jul. 02, 2017	Jul. 01, 2018	Jul. 02, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			$ 2,430,531
Other comprehensive income (loss) before reclassifications			(43,226)	$ 114,682
Amounts reclassified from accumulated other comprehensive income	$ 1,258	$ 2,231	1,897	4,740
Net current-period other comprehensive income (loss)			(41,329)	119,422
Ending Balance	2,433,981		2,433,981
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			340	(2,424)
Other comprehensive income (loss) before reclassifications			1,103	2,684
Amounts reclassified from accumulated other comprehensive income			(811)	2,477
Net current-period other comprehensive income (loss)			292	5,161
Ending Balance	632	2,737	632	2,737
Pension and Other Postretirement Benefit Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(138,808)	(136,596)
Other comprehensive income (loss) before reclassifications			188	(669)
Amounts reclassified from accumulated other comprehensive income	1,349	1,132	2,708	2,263
Net current-period other comprehensive income (loss)			2,896	1,594
Ending Balance	(135,912)	(135,002)	(135,912)	(135,002)
Foreign Currency Translation Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(126,623)	(299,697)
Other comprehensive income (loss) before reclassifications			(44,517)	112,667
Amounts reclassified from accumulated other comprehensive income			0	0
Net current-period other comprehensive income (loss)			(44,517)	112,667
Ending Balance	(171,140)	(187,030)	(171,140)	(187,030)
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(265,091)	(438,717)
Ending Balance	$ (306,420)	$ (319,295)	$ (306,420)	$ (319,295)