Retirement Benefits and Trusteed Assets (Pension Plan - Assumptions used in Determining the PBO and Net Pension Costs) (Details) - Pension plan	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Projected benefit obligation
Discount rate	3.70%	4.25%	4.50%
Rate of compensation increase	4.98%	4.65%	4.65%
Net pension costs
Discount rate	4.25%	4.50%	4.12%
Rate of compensation increase	4.65%	4.65%	4.65%
Expected long-term rate of return on plan assets	7.50%	7.75%	7.75%