Postretirement Benefits Other Than Pensions - Actuarial Assumptions of OPEB Expense (Details) - Other Postretirement Benefit Plan, Defined Benefit	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Weighted-average Discount Rate	2.96%	4.08%	3.36%
Service Cost Discount Rate	2.94%	4.16%	3.52%
Interest Cost Discount Rate	2.47%	3.69%	2.96%
Rate of Increase in Future Compensation Levels	2.20%	2.20%	2.20%