Employee Benefit Plans - Weighted-average assumptions used to determine net periodic benefit cost (Details)	12 Months Ended
Dec. 31, 2020	Dec. 26, 2019	Dec. 27, 2018
Employee Benefit Plans
Discount rate	3.10%	4.15%	3.60%
Rate of compensation increase	4.00%	4.00%	4.00%

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(2)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration