Employee Benefit Plans - Net periodic pension cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 26, 2019	Dec. 27, 2018
Net periodic pension cost:
Service cost	$ 1,095	$ 833	$ 926
Interest cost	1,371	1,485	1,364
Net amortization of prior service cost and actuarial loss	990	436	621
Total	$ 3,456	$ 2,754	$ 2,911