Accounts receivable (Allowance for doubtful accounts rollforward) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018
Receivables [Abstract]
Balance, beginning of year	$ 2,591	$ 2,912
Net additions to provision	1,097	416
Accounts receivable written off, net of recoveries	(552)	(717)
Effects of foreign exchange rate differences	(16)	(20)
Balance, end of year	$ 3,120	$ 2,591