Accounts receivable (Allowance for doubtful accounts rollforward) (Details) - USD ($)	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016
Receivables [Abstract]
Allowance for Doubtful Accounts Receivable, Beginning Balance	$ 2,947,000	$ 2,492,000
Net additions to provisions	491,000	1,013,000
Reserves from acquired business	0	0
Accounts receivable written off, net of recoveries	(596,000)	(536,000)
Effects of foreign exchange rate differences	70,000	(22,000)
Allowance for Doubtful Accounts Receivable, Ending Balance	$ 2,912,000	$ 2,947,000