Employee Retirement Plans - Assumptions (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2015
Minimum
Assumptions used in determining net periodic pension cost for the plans
Discount rates (as a percent)				3.60%
Pension plan
Assumptions used to determine the benefit obligation
Discount rate (as a percent)		3.60%	4.20%
Salaried Pension Plan
Assumptions used in determining net periodic pension cost for the plans
Rates of increase in compensation levels: Salaried (as a percent)		3.50%	3.50%
Expected long-term rate of return on assets (as a percent)		5.80%	6.50%
Assumptions used to determine the benefit obligation
Discount rate (as a percent)			3.60%
Hourly Pension Plan
Assumptions used in determining net periodic pension cost for the plans
Expected long-term rate of return on assets (as a percent)		6.50%	6.50%
Assumptions used to determine the benefit obligation
Discount rate (as a percent)		4.20%	4.50%
Other postretirement benefit cost
Assumptions used in determining net periodic pension cost for the plans
Discount rates (as a percent)	4.00%	3.40%	3.80%
Summary of healthcare cost trend rates
Ultimate healthcare cost trend rate (as a percent)	4.50%	4.50%	4.50%
Participation (as a percent)	60.00%	60.00%	60.00%
Assumed health care cost trend rate for the next fiscal year (as a percent)	6.80%	6.80%
Ultimate rate in 2028 (as a percent)	4.50%
Ultimate rate in 2027 (as a percent)		4.50%
Ultimate rate in 2026 (as a percent)			4.50%
Assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.00%	3.80%
Other postretirement benefit cost | Under 65
Summary of healthcare cost trend rates
Assumed health care cost trend rate for the next fiscal year (as a percent)			7.00%