Income taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Jan. 02, 2016
Tax Credit Carryforward, Expiration Date	Jan. 01, 2029
Valuation Allowance, Amount	$ 9,162	$ 9,090	$ 9,347
Internal Revenue Service (IRS) [Member]
Open Tax Year	2009
US | State and Local Jurisdiction [Member]
Operating Loss Carryforwards	$ 62,600	64,200
Research Tax Credit Carryforward [Member] | US
Tax Credit Carryforward, Amount	400	400
Research Tax Credit Carryforward [Member] | US State [Member]
Tax Credit Carryforward, Amount	900	800
Capital Loss Carryforward [Member] | US
Operating Loss Carryforwards	46,000	51,300
Capital Loss Carryforward [Member] | CA
Operating Loss Carryforwards	$ 27,700	$ 29,700