Postretirement Benefits - Actuarial Assumptions (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Actuarial assumptions used to determine benefit obligations:
Discount rate	2.96%	3.59%
Employee pension plan | Qualified Plan | Funded Plan
Actuarial assumptions used to determine benefit obligations:
Discount rate	2.96%	3.59%	4.47%	3.73%
Expected return on assets	6.00%	6.00%	6.75%	6.75%
Compensation increases	3.21%	3.21%	3.32%	3.32%
Actuarial assumptions used to determine net periodic benefit cost:
Discount rate	2.96%	3.59%	4.47%	3.73%
Rate of compensation increase	3.21%	3.21%	3.32%	3.32%
Supplemental employee retirement plan (SERP) | Nonqualified Plan | Unfunded plan
Actuarial assumptions used to determine benefit obligations:
Compensation increases	5.00%	5.00%	5.00%	5.00%
Pre-retirement discount rate	2.86%	3.59%	4.47%	3.73%
Post-retirement discount rate	2.86%	3.09%	3.97%	3.23%
Actuarial assumptions used to determine net periodic benefit cost:
Rate of compensation increase	5.00%	5.00%	5.00%	5.00%
Pre-retirement discount rate	2.86%	3.59%	4.47%	3.73%
Post-retirement discount rate	2.86%	3.09%	3.97%	3.23%