Advertising funds (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Advertising Funds [Abstract]
Percentage of franchisees gross retail sales, collected for advertising unds	5.00%
Accrued advertising	$ 10,600	$ 11,900
Advertising management fee	11,300	9,500	$ 9,700
Advertising expense	2,900
Advertising expense, national and regional advertising	7,500	1,700
Advertising expense, based on retail Sales as owner and operator of company-owned restaurants		600	1,300
Advertising fund contribution	$ 2,600	$ 3,400	$ 2,300