Changes in shareholders' equity - Change in accumulated other comprehensive income, net of tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 26, 2016	Jun. 28, 2015	Jun. 26, 2016	Jun. 28, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			$ 2,011,093
Amounts reclassified from accumulated other comprehensive income	$ 2,191	$ 1,403	4,777	$ 2,300
Ending Balance	2,121,568		2,121,568
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(2,491)	0
Other comprehensive income (loss) before reclassifications			(1,684)	(922)
Amounts reclassified from accumulated other comprehensive income			2,668	163
Net current-period other comprehensive income			984	(759)
Ending Balance	(1,507)	(759)	(1,507)	(759)
Pension and Other Postretirement Benefit Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(138,887)	(141,744)
Other comprehensive income (loss) before reclassifications			375	300
Amounts reclassified from accumulated other comprehensive income			2,109	2,137
Net current-period other comprehensive income			2,484	2,437
Ending Balance	(136,403)	(139,307)	(136,403)	(139,307)
Foreign Currency Translation Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(229,746)	(119,151)
Other comprehensive income (loss) before reclassifications			11,285	(61,893)
Amounts reclassified from accumulated other comprehensive income			0	0
Net current-period other comprehensive income			11,285	(61,893)
Ending Balance	(218,461)	(181,044)	(218,461)	(181,044)
Accumulated Other Comprehensive (Loss) Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			(371,124)	(260,895)
Other comprehensive income (loss) before reclassifications			9,976	(62,515)
Amounts reclassified from accumulated other comprehensive income			4,777	2,300
Net current-period other comprehensive income			14,753	(60,215)
Ending Balance	$ (356,371)	$ (321,110)	$ (356,371)	$ (321,110)