Income taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Tax Credit Carryforward, Expiration Date	Jan. 01, 2029
Valuation Allowance, Amount	$ 5,445	$ 9,162	$ 9,090
US | State and Local Jurisdiction [Member]
Operating Loss Carryforwards	15,100	62,600
Research Tax Credit Carryforward [Member] | US
Tax Credit Carryforward, Amount	1,100	400
Research Tax Credit Carryforward [Member] | US State [Member]
Tax Credit Carryforward, Amount	900	900
Capital Loss Carryforward [Member] | US
Operating Loss Carryforwards	72,000	46,000
Capital Loss Carryforward [Member] | CA
Operating Loss Carryforwards	$ 29,700	$ 27,700