Franchise fees and royalty income - Changes in Franchised and Company-Owned Points of Distribution (Detail) $ in Thousands	12 Months Ended
	Dec. 30, 2017 USD ($) distributor	Dec. 31, 2016 USD ($) distributor	Dec. 26, 2015 USD ($) distributor
Number of Franchises [Roll Forward]
Franchised points of distribution-beginning of period	20,080	19,308	18,821
Franchises opened	1,339	1,540	1,536
Franchises closed	(899)	(819)	(1,051)
Net transfers (to) from company-owned points of distribution	0	51	2
Franchised points of distribution in operation-end of period	20,520	20,080	19,308
Company-owned points of distribution-end of period	0	0	49
Total systemwide points of distribution-end of period	20,520	20,080	19,357
Franchisor Disclosure [Line Items]
Other operating income, net | $	$ 627	$ 9,381	$ 1,430
Entity Operated Units [Member]
Franchisor Disclosure [Line Items]
Other operating income, net | $		$ 7,600