Property plant and equipment (Details) - USD ($) $ in Thousands	Dec. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Cost	$ 286,768	$ 267,850
Accumulated depreciation	123,144	105,611
Net book value	163,624	162,239
Land [Member]
Property, Plant and Equipment [Line Items]
Cost	7,124	7,165
Accumulated depreciation	0	0
Net book value	7,124	7,165
Buildings [Member]
Property, Plant and Equipment [Line Items]
Cost	70,383	68,014
Accumulated depreciation	21,784	19,432
Net book value	48,599	48,582
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Cost	178,760	168,120
Accumulated depreciation	84,525	73,914
Net book value	94,235	94,206
Enterprise Software [Member]
Property, Plant and Equipment [Line Items]
Cost	14,829	10,845
Accumulated depreciation	6,651	4,179
Net book value	8,178	6,666
Office Furniture and Equipment [Member]
Property, Plant and Equipment [Line Items]
Cost	12,063	10,191
Accumulated depreciation	7,834	5,930
Net book value	4,229	4,261
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Cost	3,609	3,515
Accumulated depreciation	2,350	2,156
Net book value	$ 1,259	$ 1,359