Property plant and equipment (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 309,541	$ 286,768
Property, plant and equipment	171,032	163,624
Depreciation	21,900	21,700	$ 22,900
Capital Lease Costs [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	3,400	5,400
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	192,982	178,760
Property, plant and equipment	98,062	94,235
Inventory, parts and components, net of reserves	4,900	5,000
Machinery and Equipment [Member] | Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	19,400	23,700
Machinery and Equipment [Member] | Capital Lease Costs [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 10,100	$ 11,900