Retirement Plans - Weighted-average assumptions (Details)	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Pension Benefits
Net periodic benefit cost
Discount rate (as a percent)	3.67%	4.13%	4.66%
Expected long-term rate of return on assets (as a percent)	7.58%	7.57%	7.58%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.49%
Benefit obligations at year-end
Discount rate (as a percent)	4.24%	3.66%	4.13%
Rate of compensation increase (as a percent)	3.50%	3.50%	3.50%
Interest crediting rate for cash balance plans (as a percent)	5.25	5.25	5.25
Postretirement Benefits Other Than Pensions
Net periodic benefit cost
Discount rate (as a percent)	3.50%	4.00%	4.50%
Benefit obligations at year-end
Discount rate (as a percent)	4.25%	3.50%	4.00%