Intangible assets (Major Components) (Details) - USD ($) $ in Thousands	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Cost	$ 212,764	$ 213,095
Accumulated amortization	51,789	41,036
Net book value	160,975	172,059	$ 183,524
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	210,845	211,176
Accumulated amortization	49,937	39,274
Net book value	160,908	171,902	183,258
Patents, Trademarks and Other
Finite-Lived Intangible Assets [Line Items]
Cost	1,919	1,919
Accumulated amortization	1,852	1,762
Net book value	$ 67	$ 157	$ 266