Shareholders' equity - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 28, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes		$ 712,452	$ 622,936	$ 990,696
Income tax effect		(216,979)	(202,314)	(234,471)
Total reclassifications, net of tax	$ 19,910	4,940		27,569
Retirement Plans
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications, net of tax	$ 19,910	4,940	19,910	27,569
Retirement Plans | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service cost		96	1,176	(4,082)
Amortization of actuarial loss		7,972	31,357	46,489
Income before income taxes		8,068	32,533	42,407
Income tax effect		$ (3,128)	$ (12,623)	$ (14,838)