Retirement Benefits and Trusteed Assets (OPEB - Assumptions used in Determining the PBO and Net Pension Costs) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Ultimate health care trend rate			4.50%
Other postretirement benefit plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate, accumulated postretirement benefit obligations	4.25%	4.50%	4.10%
Health care trend rate pre- 65, accumulated postretirement benefit obligation	6.50%	6.25%	7.50%
Health care trend post- 65, accumulated postretirement benefit obligation	6.75%	6.75%	6.50%
Ultimate health care trend rate	4.50%	4.50%	4.50%
Discount rate, other postretirement benefit costs	4.50%	4.10%	4.95%
Expected long-term rate of return on plan assets, other postretirement benefit costs	8.00%	8.00%	8.00%
Health care trend rate pre- 65, other postretirement benefit costs	6.25%	7.50%	7.50%
Health care trend post- 65, other postretirement benefit costs	6.75%	6.50%	6.50%