Other Postretirement Benefits - Changes in the Benefit Obligation and Plan Assets and Other Amounts (Details) - Other Postretirement Benefit Plans - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2020	Dec. 29, 2019	Dec. 30, 2018
Change in benefit obligation
Benefit obligation at beginning of year	$ 42,803	$ 46,037
Service cost	29	27	$ 21
Interest cost	1,026	1,602	1,476
Plan participants’ contributions	2,820	3,835
Actuarial loss	4,044	296
Benefits paid	(7,414)	(8,994)
Benefit obligation at end of year	43,308	42,803	$ 46,037
Change in plan assets
Fair value of plan assets at beginning of year	0
Employer contributions	4,594	5,159
Plan participants’ contributions	2,820	3,835
Benefits paid	(7,414)	(8,994)
Fair value of plan assets at end of year	0	0
Amount recognized in the Consolidated Balance Sheets
Current liabilities	(4,618)	(5,115)
Noncurrent liabilities	(38,690)	(37,688)
Net amount recognized	(43,308)	(42,803)
Amount recognized in accumulated other comprehensive loss
Actuarial loss	26,785	25,793
Prior service credit	(3,466)	(7,691)
Total	$ 23,319	$ 18,102