Retirement Plans and Other Postretirement Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted-average assumptions used to determine benefit obligations
Rate of compensation increase (as a percent)	4.00%	4.00%
Initial pre-65 healthcare cost trend rate (as a percent)	6.50%	7.00%
Initial pre-65 ultimate health care cost trend rate (as a percent)	4.75%	4.75%
Number of years to ultimate trend rate (pre-65 participants)	5 years	6 years
Initial post-65 healthcare cost trend rate (as a percent)	2.00%	4.75%
Interest crediting rate – cash balance pension plans	4.50%	4.50%
Weighted-average assumptions used to determine net periodic benefit costs
Initial pre-65 health care cost trend rate (as a percent)	7.00%	7.00%	7.00%
Initial pre-65 ultimate healthcare cost trend rate (as a percent)	4.75%	4.75%	4.75%
Number of years to ultimate trend rate (pre-65 participants)	5 years	7 years	8 years
Initial post-65 health care cost trend rate (as a percent)	4.75%	4.75%	4.75%
Interest crediting rate – cash balance pension plans	4.50%	4.50%	4.50%
Pension Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	2.53%	3.30%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	3.30%	4.34%	3.65%
Rate of compensation increase (as a percent)	4.00%	4.00%	4.00%
Expected long-term return on plan assets (as a percent)	5.75%	6.25%	6.05%
Other Benefits
Weighted-average assumptions used to determine benefit obligations
Discount rate (as a percent)	2.63%	3.42%
Weighted-average assumptions used to determine net periodic benefit costs
Discount rate (as a percent)	3.42%	4.39%	3.71%
Expected long-term return on plan assets (as a percent)	4.85%	5.40%	5.40%