Shareholders' equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 28, 2014	Dec. 29, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of actuarial loss	$ 31,357	$ 46,489	$ 64,381
Income before income taxes	622,936	990,696	111,818
Income tax effect	(202,314)	(234,471)	(13,122)
Total reclassifications, net of tax	19,910	27,569	41,057
Retirement Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	19,910	27,569	$ 41,057
Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service cost (credit)	1,176	(4,082)
Amortization of actuarial loss	31,357	46,489
Income before income taxes	32,533	42,407
Income tax effect	$ (12,623)	$ (14,838)