Employee Retirement Plans and Postretirement Benefits Weighted Average Assumptions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]
Effect on total of service and interest cost components, one-percent point increase (unfavorable)	$ (3.2)
Effect on total of service and interest cost components, one-percent point decrease favorable	2.8
Effect on postretirement benefit obligation, one-percent point increase (unfavorable)	(60.2)
Effect on postretirement benefit obligation, one-percent point decrease favorable	$ 51.9
Pension
Defined Benefit Plan Disclosure [Line Items]
Settlement discount rate	3.36%	3.72%	3.70%
Rate of compensation increase (as a percent)	2.00%	2.00%	2.50%
Expected return on plan assets (as a percent)	4.83%	5.15%	5.46%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.01%	3.36%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase	2.00%	2.00%
OPEB
Defined Benefit Plan Disclosure [Line Items]
Settlement discount rate	3.76%	3.59%	4.15%
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate	3.34%	3.76%
OPEB | Maximum
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate	7.00%	7.70%	7.70%
Year that rate reaches ultimate trend rate	2017	2016	2015
OPEB | Minimum
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate	4.50%	4.50%	4.50%
Year that rate reaches ultimate trend rate	2037	2028	2028