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Income taxes (Narrative) (Details) - USD ($)
$ in Thousands
Jan. 01, 2022
Jan. 02, 2021
Dec. 28, 2019
Income Tax Disclosure [Line Items]      
Deferred tax assets, valuation allowance $ 5,267 $ 4,284 $ 6,219
U.S. [Member] | Scientific research investment tax credits [Member] | Federal [Member]      
Income Tax Disclosure [Line Items]      
Tax credit carryforward, amount 1,600 1,500  
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 64,800 37,100  
U.S. [Member] | Capital loss carryforward [Member] | State [Member]      
Income Tax Disclosure [Line Items]      
Operating loss carryforwards 3,300 8,700  
Canada [Member] | Capital loss carryforward [Member]      
Income Tax Disclosure [Line Items]      
Operating loss carryforwards 27,800 27,900  
Canada [Member] | Capital loss carryforward [Member] | Foreign tax authority [Member]      
Income Tax Disclosure [Line Items]      
Operating loss carryforwards $ 6,000 $ 2,800