Pension Benefits - Schedule of Allocation of Plan Assets (Details) - Pension Plan	12 Months Ended
	Dec. 27, 2020
Pension Benefits
Percent of funded status policy maximum range	102.50%
Long Duration Assets
Pension Benefits
Target allocation percentage of assets, range minimum	77.50%
Target allocation percentage of assets, range maximum	82.50%
Public Equity
Pension Benefits
Asset allocation, actual percentage	87.00%
High-Yield Fixed Income
Pension Benefits
Asset allocation, actual percentage	0.00%
Alternatives
Pension Benefits
Asset allocation, actual percentage	11.00%
Cash
Pension Benefits
Asset allocation, actual percentage	2.00%
Long Duration Fixed Income
Pension Benefits
Asset allocation, actual percentage	78.00%
Long Duration and Return-Seeking Assets, Public Equity
Pension Benefits
Asset allocation, actual percentage	19.00%
Long Duration and Return-Seeking Assets, Growth Fixed Income
Pension Benefits
Asset allocation, actual percentage	0.00%
Long Duration and Return-Seeking Assets, Alternatives
Pension Benefits
Asset allocation, actual percentage	2.00%
Long Duration and Return-Seeking Assets, Cash
Pension Benefits
Asset allocation, actual percentage	1.00%
Hedging Assets
Pension Benefits
Asset allocation, actual percentage	78.00%
Return-Seeking Assets
Pension Benefits
Target allocation percentage of assets, range minimum	17.50%
Target allocation percentage of assets, range maximum	22.50%
Asset allocation, actual percentage	22.00%
Cash and Cash Equivalents
Pension Benefits
Asset allocation, actual percentage	0.00%
Minimum | Public Equity
Pension Benefits
Asset allocation, target percentage range	70.00%
Minimum | High-Yield Fixed Income
Pension Benefits
Asset allocation, target percentage range	0.00%
Minimum | Alternatives
Pension Benefits
Asset allocation, target percentage range	0.00%
Minimum | Cash
Pension Benefits
Asset allocation, target percentage range	0.00%
Minimum | Long Duration Fixed Income
Pension Benefits
Asset allocation, target percentage range	77.50%
Minimum | Long Duration and Return-Seeking Assets, Public Equity
Pension Benefits
Asset allocation, target percentage range	12.30%
Minimum | Long Duration and Return-Seeking Assets, Growth Fixed Income
Pension Benefits
Asset allocation, target percentage range	0.00%
Minimum | Long Duration and Return-Seeking Assets, Alternatives
Pension Benefits
Asset allocation, target percentage range	0.00%
Minimum | Long Duration and Return-Seeking Assets, Cash
Pension Benefits
Asset allocation, target percentage range	0.00%
Minimum | Hedging Assets
Pension Benefits
Asset allocation, target percentage range	75.00%
Minimum | Return-Seeking Assets
Pension Benefits
Asset allocation, target percentage range	10.00%
Minimum | Cash and Cash Equivalents
Pension Benefits
Asset allocation, target percentage range	0.00%
Maximum | Public Equity
Pension Benefits
Asset allocation, target percentage range	100.00%
Maximum | High-Yield Fixed Income
Pension Benefits
Asset allocation, target percentage range	15.00%
Maximum | Alternatives
Pension Benefits
Asset allocation, target percentage range	15.00%
Maximum | Cash
Pension Benefits
Asset allocation, target percentage range	10.00%
Maximum | Long Duration Fixed Income
Pension Benefits
Asset allocation, target percentage range	82.50%
Maximum | Long Duration and Return-Seeking Assets, Public Equity
Pension Benefits
Asset allocation, target percentage range	22.50%
Maximum | Long Duration and Return-Seeking Assets, Growth Fixed Income
Pension Benefits
Asset allocation, target percentage range	3.00%
Maximum | Long Duration and Return-Seeking Assets, Alternatives
Pension Benefits
Asset allocation, target percentage range	3.00%
Maximum | Long Duration and Return-Seeking Assets, Cash
Pension Benefits
Asset allocation, target percentage range	2.00%
Maximum | Hedging Assets
Pension Benefits
Asset allocation, target percentage range	90.00%
Maximum | Return-Seeking Assets
Pension Benefits
Asset allocation, target percentage range	25.00%
Maximum | Cash and Cash Equivalents
Pension Benefits
Asset allocation, target percentage range	5.00%