Employee Benefit Plans - Assumptions Used to Determine Projected Postretirement Benefit Obligations and Postretirement Costs (Details)	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Weighted-average assumptions used to determine projected pension benefit obligations
Discount rate	2.65%	3.50%	4.45%
Health care cost trend rate for medical benefits assumed for next year (pre-65)	6.75%	7.00%	7.25%
Health care cost trend rate for medical benefits assumed for next year (post-65)	5.00%	5.25%	5.50%
Ultimate trend rate (pre-65)	4.50%	4.50%	4.50%
Ultimate trend rate (post-65)	4.50%	4.50%	4.50%
Beginning discount rate	3.50%	4.45%	3.85%