Pension, Retiree Medical and Savings Plans Pension, Retiree Medical and Savings Plans (Policies)	12 Months Ended
	Dec. 26, 2020
Retirement Benefits [Abstract]
Pension and Other Postretirement Plans, Policy [Policy Text Block]	Gains and losses resulting from actual experience differing from our assumptions, including the difference between the actual return on plan assets and the expected return on plan assets, as well as changes in our assumptions, are determined at each measurement date. These differences are recognized as a component of net gain or loss in accumulated other comprehensive loss. If this net accumulated gain or loss exceeds 10% of the greater of the market-related value of plan assets or plan obligations, a portion of the net gain or loss is included in other pension and retiree medical benefits (expense)/income for the following year based upon the average remaining service life for participants in Plan A (approximately 10 years) and retiree medical (approximately 8 years), or the remaining life expectancy for participants in Plan I (approximately 23 years). In 2021, we expect the average remaining service life for participants in Plan A to be approximately 9 years, the remaining life expectancy for participants in Plan I to be approximately 27 years and the average remaining service life for participants in Plan H to be approximately 11 years. The cost or benefit of plan changes that increase or decrease benefits for prior employee service (prior service cost/(credit)) is included in other pension and retiree medical benefits (expense)/income on a straight-line basis over the average remaining service life for participants in both Plan A and Plan H, except that prior service cost/(credit) for salaried participants subject to the freeze will be amortized on a straight-line basis over the period up to the effective date of the freeze, or the remaining life expectancy for participants in Plan I.

Disclosure of accounting policy for pension and other postretirement benefit plans. This accounting policy may address (1) the types of plans sponsored by the entity, and the benefits provided by each plan (2) groups that participate in (or are covered by) each plan (3) how plan assets, liabilities and expenses are measured, including the use of any actuaries and (4) significant assumptions used by the entity to value plan assets and liabilities and how such assumptions are derived.