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, 2020	Dec. 31, 2019	Dec. 31, 2018
Projected benefit obligation
Discount rate	2.57%	3.28%	4.40%
Rate of compensation increase	3.80%	3.85%	3.85%
Cash balance interest crediting rate	2.00%	3.30%	3.70%
Net pension costs
Discount rate	3.28%	4.40%	3.70%
Rate of compensation increase	3.85%	3.85%	3.85%
Expected long-term rate of return on plan assets	7.10%	7.30%	7.50%
Cash balance interest crediting rate	3.30%	3.70%	3.70%