Pension Benefit and Retirement Health and Life Insurance Benefits (Assumptions Used) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.70%	4.25%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.70%	4.25%
Expected long-term rate of return on plan assets	4.94%	5.51%
Retirement Health and Life Insurance Benefits
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.25%	3.25%
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate	3.25%	3.00%
Expected long-term rate of return on plan assets	0.00%	0.00%