Other Postretirement Benefits - Changes in the Benefit Obligations Recognized in Other Comprehensive Income (Details) - Other Postretirement Benefit Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2020	Dec. 29, 2019	Dec. 30, 2018
Other Postretirement Benefits
Net actuarial loss/(gain)	$ 4,044	$ 296	$ (4,905)
Amortization of loss	(3,051)	(3,375)	(4,735)
Amortization of prior service credit	4,225	4,766	6,157
Total recognized in other comprehensive income	5,218	1,687	(3,483)
Net periodic postretirement benefit cost/(income)	(119)	238	75
Total recognized in net periodic postretirement benefit cost/(income) and other comprehensive loss/(income)	$ 5,099	$ 1,925	$ (3,408)