Postretirement Benefits Other Than Pensions - Actuarial Assumptions of OPEB Accumulated Benefit Obligation (Details) - Other Postretirement Benefit Plan, Defined Benefit [Member]	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Discount Rate	3.36%	3.60%
Rate of Increase in Future Compensation Levels	2.20%	2.60%