Pension Benefits - Changes in Benefit Obligation and Plan Assets (Details) - Pension Plan - 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	$ 1,908,035	$ 1,714,464
Service cost	10,548	5,231	$ 10,065
Interest cost	50,311	67,255	60,153
Amendments	0	706
Actuarial loss	174,288	223,978
Curtailments	(562)	(373)
Settlements	(236,282)	0
Benefits paid	(97,877)	(103,209)
Effects of change in currency conversion	144	(17)
Benefit obligation at end of year	1,808,605	1,908,035	1,714,464
Change in plan assets
Fair value of plan assets at beginning of year	1,648,667	1,410,151
Actual return on plan assets	245,606	315,148
Employer contributions	25,107	26,577
Settlements	(236,282)	0
Benefits paid	(97,877)	(103,209)
Fair value of plan assets at end of year	1,585,221	1,648,667	1,410,151
Amount recognized in the Consolidated Balance Sheets
Noncurrent Assets	54,950	0
Current liabilities	(16,990)	(17,147)
Noncurrent liabilities	(261,344)	(242,221)
Net amount recognized	(223,384)	(259,368)
Amount recognized in accumulated other comprehensive loss
Actuarial loss	602,619	715,391
Prior service credit	(14,255)	(16,149)
Total	588,364	699,242
Qualified Plan
Change in benefit obligation
Benefit obligation at beginning of year	1,660,287	1,491,398
Service cost	10,429	5,113	9,986
Interest cost	43,710	58,835	52,770
Amendments	0	0
Actuarial loss	153,136	191,104
Curtailments	0	0
Settlements	(236,282)	0
Benefits paid	(82,268)	(86,163)
Effects of change in currency conversion	0	0
Benefit obligation at end of year	1,549,012	1,660,287	1,491,398
Change in plan assets
Fair value of plan assets at beginning of year	1,648,667	1,410,151
Actual return on plan assets	245,606	315,148
Employer contributions	9,498	9,531
Settlements	(236,282)	0
Benefits paid	(82,268)	(86,163)
Fair value of plan assets at end of year	1,585,221	1,648,667	1,410,151
Amount recognized in the Consolidated Balance Sheets
Noncurrent Assets	54,950	0
Current liabilities	0	0
Noncurrent liabilities	(18,741)	(11,620)
Net amount recognized	36,209	(11,620)
Amount recognized in accumulated other comprehensive loss
Actuarial loss	464,922	592,774
Prior service credit	(14,897)	(16,842)
Total	450,025	575,932
Nonqualified Plan
Change in benefit obligation
Benefit obligation at beginning of year	247,748	223,066
Service cost	119	118	79
Interest cost	6,601	8,420	7,383
Amendments	0	706
Actuarial loss	21,152	32,874
Curtailments	(562)	(373)
Settlements	0	0
Benefits paid	(15,609)	(17,046)
Effects of change in currency conversion	144	(17)
Benefit obligation at end of year	259,593	247,748	223,066
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	15,609	17,046
Settlements	0	0
Benefits paid	(15,609)	(17,046)
Fair value of plan assets at end of year	0	0	$ 0
Amount recognized in the Consolidated Balance Sheets
Noncurrent Assets	0	0
Current liabilities	(16,990)	(17,147)
Noncurrent liabilities	(242,603)	(230,601)
Net amount recognized	(259,593)	(247,748)
Amount recognized in accumulated other comprehensive loss
Actuarial loss	137,697	122,617
Prior service credit	642	693
Total	$ 138,339	$ 123,310