Changes in shareholders' equity Change in accumulated other comprehensive income , net of tax (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 28, 2015	Jun. 29, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss)	$ (260,895)	$ (110,855)
Other comprehensive income (loss) before reclassifications	(62,515)	5,199
Amounts reclassified from accumulated other comprehensive (loss) income	2,300	1,408
Net current-period other comprehensive income (loss)	(60,215)	6,607
Ending balance, accumulated other comprehensive income (loss)	(321,110)	(104,248)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss)	0	0
Other comprehensive income (loss) before reclassifications	(922)	233
Amounts reclassified from accumulated other comprehensive (loss) income	163	(90)
Net current-period other comprehensive income (loss)	(759)	143
Ending balance, accumulated other comprehensive income (loss)	(759)	143
Pension and Other Postretirement Benefits Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss)	(141,744)	(97,037)
Other comprehensive income (loss) before reclassifications	300	(256)
Amounts reclassified from accumulated other comprehensive (loss) income	2,137	1,498
Net current-period other comprehensive income (loss)	2,437	1,242
Ending balance, accumulated other comprehensive income (loss)	(139,307)	(95,795)
Foreign Currency Translation Adjustment
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance, accumulated other comprehensive income (loss)	(119,151)	(13,818)
Other comprehensive income (loss) before reclassifications	(61,893)	5,222
Amounts reclassified from accumulated other comprehensive (loss) income	0	0
Net current-period other comprehensive income (loss)	(61,893)	5,222
Ending balance, accumulated other comprehensive income (loss)	$ (181,044)	$ (8,596)