Employee Benefit Plans - pre-tax change in the benefit obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 26, 2019
Employee Benefit Plans
Net actuarial loss	$ 3,806	$ 7,317
Amortization of the net actuarial loss	(1,055)	(499)
Amortization of the prior service credit	64	63
Total	$ 2,815	$ 6,881