Pension, Retiree Medical and Savings Plans Pension, Retiree Medical and Savings Plans (Target Asset Allocations) (Details)	Dec. 26, 2020	Dec. 28, 2019
Fixed Income
Target Asset Allocation [Line Items]
Target investment allocation percentage		47.00%
U.S. Equity
Target Asset Allocation [Line Items]
Target investment allocation percentage		29.00%
International Equity
Target Asset Allocation [Line Items]
Target investment allocation percentage		20.00%
Real Estate Funds [Member]
Target Asset Allocation [Line Items]
Target investment allocation percentage		4.00%
Forecast [Member] | Fixed Income
Target Asset Allocation [Line Items]
Target investment allocation percentage	50.00%
Forecast [Member] | U.S. Equity
Target Asset Allocation [Line Items]
Target investment allocation percentage	25.00%
Forecast [Member] | International Equity
Target Asset Allocation [Line Items]
Target investment allocation percentage	21.00%
Forecast [Member] | Real Estate Funds [Member]
Target Asset Allocation [Line Items]
Target investment allocation percentage	4.00%