Property plant and equipment (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 286,768	$ 267,850
Property, Plant and Equipment	163,624	162,239
Depreciation	21,700	22,900	$ 16,100
Capital Lease Costs [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment	5,400	10,700
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	178,760	168,120
Property, Plant and Equipment	94,235	94,206
Inventory, Parts and Components, Net of Reserves	5,000	6,100
Machinery and Equipment [Member] | Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	23,700	13,700
Machinery and Equipment [Member] | Capital Lease Costs [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 11,900	$ 11,900