Postretirement Benefits Other Than Pensions - Actuarial Assumptions of OPEB Expense (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	3.61%	3.70%	3.40%
Weighted-average Discount Rate	3.79%	4.21%	0.00%
Interest Cost Discount Rate	2.92%	2.90%	0.00%
Rate of Increase in Future Compensation Levels	2.60%	2.64%	2.68%