Pension, Retiree Medical and Savings Plans (Level 3 Assets) (Details)	12 Months Ended
Dec. 26, 2020	Dec. 28, 2019
Change in fair value of plan assets
Minimum Real estate notice period	45 days
Corporate Bonds
Change in fair value of plan assets
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage	30.00%	28.00%

Percentage of investment to total investment within defined benefit plan asset category.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Name:	us-gaap_DefinedBenefitPlanPlanAssetsInvestmentWithinPlanAssetCategoryPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant