Income taxes (Narrative) (Details) - USD ($) $ in Thousands	Dec. 28, 2019	Dec. 29, 2018	Dec. 30, 2017
Income Tax Disclosure [Line Items]
Deferred tax assets, valuation allowance	$ 6,219	$ 5,445	$ 9,162
U.S. [Member] | Scientific research investment tax credits [Member] | Federal [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforward, amount	600	1,100
U.S. [Member] | Research and development tax credits [Member] | State [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforward, amount	900	900
U.S. [Member] | Capital loss carryforward [Member] | Federal [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	78,000	72,000
U.S. [Member] | Capital loss carryforward [Member] | State [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	14,400	15,100
Canada [Member] | Capital loss carryforward [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$ 28,900	$ 29,700