Other Postretirement Benefits - Other Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2020	Dec. 29, 2019	Dec. 30, 2018	Oct. 31, 2020
Other Postretirement Benefits
Benefit obligation				$ 236,300
Other Postretirement Benefit Plans
Other Postretirement Benefits
Additional postretirement costs	$ 16,000	$ 15,000	$ 16,000
Benefit obligation	43,308	42,803	$ 46,037
Actuarial loss	4,044	296
Postemployment benefits liability	$ 8,600	$ 9,500