Allowance for doubtful accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Provision for (recovery of) doubtful accounts, net	$ (457)	$ (53)	$ (3,343)
Accounts receivable
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	4,778	5,627	3,882
Provision for (recovery of) doubtful accounts, net	(123)	1,202	3,705
Write-offs and other	968	2,051	1,960
Ending balance	3,933	4,778	5,627
Short-term notes and other receivables
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	339	1,007	1,278
Provision for (recovery of) doubtful accounts, net	(264)	(189)	(117)
Write-offs and other	0	479	154
Ending balance	603	339	1,007
Long-term notes and other receivables
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning balance	2,088	4,075	3,947
Provision for (recovery of) doubtful accounts, net	(70)	(960)	(245)
Write-offs and other	335	1,027	373
Ending balance	$ 1,823	$ 2,088	$ 4,075