Postretirement Benefits Other Than Pensions - Actuarial Assumptions of OPEB Expense (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member]	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	3.36%	3.61%	3.70%
Weighted-average Discount Rate	3.52%	3.79%	4.21%
Interest Cost Discount Rate	2.96%	2.92%	2.90%
Rate of Increase in Future Compensation Levels	2.20%	2.60%	2.64%