Property plant and equipment (Details) - USD ($) $ in Thousands	Dec. 29, 2018	Dec. 30, 2017
Property, Plant and Equipment [Line Items]
Cost	$ 309,541	$ 286,768
Accumulated depreciation	138,509	123,144
Net book value	171,032	163,624
Land [Member]
Property, Plant and Equipment [Line Items]
Cost	7,075	7,124
Accumulated depreciation	0	0
Net book value	7,075	7,124
Buildings [Member]
Property, Plant and Equipment [Line Items]
Cost	73,792	70,383
Accumulated depreciation	24,059	21,784
Net book value	49,733	48,599
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Cost	192,982	178,760
Accumulated depreciation	94,920	84,525
Net book value	98,062	94,235
Enterprise Software [Member]
Property, Plant and Equipment [Line Items]
Cost	20,996	14,829
Accumulated depreciation	8,878	6,651
Net book value	12,118	8,178
Office Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Cost	11,505	12,063
Accumulated depreciation	8,472	7,834
Net book value	3,033	4,229
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Cost	3,191	3,609
Accumulated depreciation	2,180	2,350
Net book value	$ 1,011	$ 1,259