Shareholders' equity Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 28, 2014	Dec. 29, 2013	Dec. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of actuarial loss	$ 46,489	$ 64,381	$ 55,372
Settlement charge	0	3,077	7,946
Income before income taxes	1,356,060	559,113	670,407
Provision for income taxes	(225,600)	(113,200)	(195,400)
Total reclassifications, net of tax	27,569	41,057
Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	27,569	41,057
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service credit	(4,082)	(1,599)
Amortization of actuarial loss	46,489	64,381
Settlement charge	0	3,077
Income before income taxes	42,407	65,859
Provision for income taxes	$ (14,838)	$ (24,802)