Pension Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2020	Dec. 27, 2020	Dec. 27, 2020	Dec. 29, 2019	Dec. 30, 2018
Pension Benefits
Benefit obligation	$ 236,300
Pension settlement expense	$ 80,600	$ (80,600)
Pension Plan
Pension Benefits
Service cost			$ 10,548	$ 5,231	$ 10,065
Interest cost			50,311	67,255	60,153
Expected return on plan assets			(67,146)	(80,877)	(82,327)
Amortization and other costs			27,959	23,020	31,916
Amortization of prior service (credit)/cost			(1,894)	(1,932)	(1,945)
Effect of settlement/curtailment			80,079	(373)	221
Net periodic pension (income)/cost			99,857	12,324	18,083
Benefit obligation		1,808,605	1,808,605	1,908,035	1,714,464
Qualified Plan | Pension Plan
Pension Benefits
Service cost			10,429	5,113	9,986
Interest cost			43,710	58,835	52,770
Expected return on plan assets			(67,146)	(80,877)	(82,327)
Amortization and other costs			21,887	18,639	26,802
Amortization of prior service (credit)/cost			(1,945)	(1,945)	(1,945)
Effect of settlement/curtailment			80,641	0	0
Net periodic pension (income)/cost			87,576	(235)	5,286
Benefit obligation		1,549,012	1,549,012	1,660,287	1,491,398
Nonqualified Plan | Pension Plan
Pension Benefits
Service cost			119	118	79
Interest cost			6,601	8,420	7,383
Expected return on plan assets			0	0	0
Amortization and other costs			6,072	4,381	5,114
Amortization of prior service (credit)/cost			51	13	0
Effect of settlement/curtailment			(562)	(373)	221
Net periodic pension (income)/cost			12,281	12,559	12,797
Benefit obligation		$ 259,593	$ 259,593	$ 247,748	$ 223,066