Significant accounting policies - Property, plant and equipment (Details)	12 Months Ended
	Dec. 28, 2019
Buildings and improvements | Minimum
Property, Plant and Equipment
Fixed assets (useful life)	30 years
Buildings and improvements | Maximum
Property, Plant and Equipment
Fixed assets (useful life)	40 years
Machinery, equipment and vehicles | Minimum
Property, Plant and Equipment
Fixed assets (useful life)	2 years
Machinery, equipment and vehicles | Maximum
Property, Plant and Equipment
Fixed assets (useful life)	20 years