Retirement Benefits and Trusteed Assets (OPEB - Assumptions used in Determining the PBO and Net Pension Costs) (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated postretirement benefit obligation
Ultimate health care trend rate			4.50%
Other postretirement benefit costs
Ultimate health care trend rate			4.50%
Other postretirement benefit plan
Accumulated postretirement benefit obligation
Discount rate	2.58%	3.29%	4.40%
Health care trend rate pre- 65	6.75%	6.75%	6.75%
Health care trend post- 65	7.25%	7.25%	7.25%
Ultimate health care trend rate	4.50%	4.50%	4.50%
Other postretirement benefit costs
Discount rate	3.29%	4.40%	3.70%
Expected long-term rate of return on plan assets	7.20%	7.30%	7.75%
Health care trend rate pre- 65	6.75%	6.75%	6.75%
Health care trend post- 65	7.25%	7.25%	7.25%
Ultimate health care trend rate	4.50%	4.50%	4.50%