Shareholders' equity Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 28, 2014	Dec. 29, 2013	Dec. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	$ (494,055)	$ (701,141)	$ (595,839)
Other comprehensive income before reclassifications	(312,283)
Other comprehensive income (loss)		166,029	(105,302)
Amounts reclassified from accumulated other comprehensive income	27,569	41,057
Balance at end of year	(778,769)	(494,055)	(701,141)
Accumulated Defined Benefit Plans Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	(921,232)	(1,119,263)	(995,854)
Other comprehensive income before reclassifications	(276,219)
Other comprehensive income (loss)		156,974	(123,409)
Amounts reclassified from accumulated other comprehensive income	27,569	41,057
Balance at end of year	(1,169,882)	(921,232)	(1,119,263)
Accumulated Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance at beginning of year	427,177	418,122	400,015
Other comprehensive income before reclassifications	(36,064)
Other comprehensive income (loss)		9,055	18,107
Amounts reclassified from accumulated other comprehensive income	0	0
Balance at end of year	$ 391,113	$ 427,177	$ 418,122