Derivative instruments and hedging transactions - Derivative Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016	Dec. 26, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Amount of net gain (loss) reclassified into earnings, interest rate swaps	$ 1,922	$ 2,181	$ (2,140)
Amount of net gain (loss) reclassified into earnings, income tax effect	(778)	(882)	867
Amount of net gain (loss) reclassified into earnings, net of income taxes	1,144	1,299	(1,273)
Total effect on other comprehensive income (loss), interest expense	(1,922)	(2,181)	(2,140)
Total effect on other comprehensive income (loss), provision (benefit) for income taxes	778	882	867
Total effect on other comprehensive income (loss), net of income taxes	$ (1,144)	$ (1,299)	$ (1,273)