Benefit Plans - Significant Assumptions Used, Postretirement Benefits (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 29, 2019	Dec. 30, 2018
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation discount rate at measurement date	2.70%	3.32%	4.41%
Net periodic postretirement benefit cost discount rate for fiscal year	3.32%	4.41%	3.72%
Pre Medicare
Defined Benefit Plan Disclosure [Line Items]
Weighted average healthcare cost trend rate	6.53%	7.13%	7.82%
Trend rate graded down to ultimate rate	4.50%	4.50%	4.50%
Ultimate rate year	2028	2026	2025
Post Medicare
Defined Benefit Plan Disclosure [Line Items]
Weighted average healthcare cost trend rate	6.73%	7.11%	7.74%
Trend rate graded down to ultimate rate	4.50%	4.50%	4.50%
Ultimate rate year	2028	2026	2025