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Accounts receivable (Allowance for doubtful accounts rollforward) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 29, 2018
Dec. 30, 2017
Receivables [Abstract]    
Allowance for Doubtful Accounts Receivable, Beginning Balance $ 2,912 $ 2,947
Net additions to provision 416 491
Accounts receivable written off, net of recoveries (717) (596)
Effects of foreign exchange rate differences (20) 70
Allowance for Doubtful Accounts Receivable, Ending Balance $ 2,591 $ 2,912