Shareholders' equity - Change in accumulated other comprehensive income, net of tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Oct. 01, 2017	Sep. 30, 2018	Oct. 01, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			$ 2,430,531
Other comprehensive income (loss) before reclassifications			(27,187)	$ 158,345
Amounts reclassified from accumulated other comprehensive income	$ 1,214	$ 182	3,111	4,922
Net current-period other comprehensive income (loss)			(24,076)	163,267
Ending Balance	2,507,065		2,507,065
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			340	(2,424)
Other comprehensive income (loss) before reclassifications			2,816	3,383
Amounts reclassified from accumulated other comprehensive income			(873)	1,535
Net current-period other comprehensive income (loss)			1,943	4,918
Ending Balance	2,283	2,494	2,283	2,494
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			127	(1,050)
Amounts reclassified from accumulated other comprehensive income	1,276	1,124	3,984	3,387
Net current-period other comprehensive income (loss)			4,111	2,337
Ending Balance	(134,697)	(134,259)	(134,697)	(134,259)
Foreign Currency Translation Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(126,623)	(299,697)
Other comprehensive income (loss) before reclassifications			(30,130)	156,012
Amounts reclassified from accumulated other comprehensive income			0	0
Net current-period other comprehensive income (loss)			(30,130)	156,012
Ending Balance	(156,753)	(143,685)	(156,753)	(143,685)
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(265,091)	(438,717)
Ending Balance	$ (289,167)	$ (275,450)	$ (289,167)	$ (275,450)