Postretirement Benefits Other Than Pensions - Actuarial Assumptions of OPEB Expense (Details) - Other Postretirement Benefit Plan, Defined Benefit	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Weighted-average Discount Rate	4.08%	3.36%	3.61%
Service Cost Discount Rate	4.16%	3.52%	3.79%
Interest Cost Discount Rate	3.69%	2.96%	2.92%
Rate of Increase in Future Compensation Levels	2.20%	2.20%	2.60%