Intangible assets (Major Components) (Details) - USD ($) $ in Thousands	Dec. 28, 2019	Dec. 29, 2018
Finite-Lived Intangible Assets [Line Items]
Cost	$ 212,076	$ 212,764
Accumulated amortization	62,067	51,789
Net book value	150,009	160,975
Customer relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	210,157	210,845
Accumulated amortization	60,173	49,937
Net book value	149,984	160,908
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Cost	1,919	1,919
Accumulated amortization	1,894	1,852
Net book value	$ 25	$ 67