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Pension and Postretirement Health and Life Benefits (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Actuarial (gain) loss from remeasurement, net   $ (1.4) $ 29.6
Defined Contribution Plan, Cost Recognized   3.0  
Scenario, Forecast [Member]      
Expected defined benefit plan contributions for next fiscal year for qualified plans $ 1.1    
Expected defined benefit plan contributions for next fiscal year for non-qualified plans 0.6    
Expected defined benefit plan contributions for next fiscal year for postretirement health and life benefit plans $ 13.8    
Pension Plan, Defined Benefit [Member]      
Defined Benefit Plan, Accumulated Benefit Obligation   814.6 832.8
Defined Benefit Plan, Net Periodic Benefit Cost   $ 8.1 (4.5)
Approximate Number of Plan Participants   5,000  
Defined Benefit Plan, Contributions by Employer   $ 4.1 8.0
Actuarial (gain) loss from remeasurement, net   $ 19.1 $ (55.2)
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate   4.10% 4.50%
Defined Benefit Plan, Expected Return on Plan Assets   $ 42.0 $ 47.4
Other Postretirement Benefit Plan, Defined Benefit [Member]      
Defined Benefit Plan, Net Periodic Benefit Cost   $ (1.9) (21.9)
Approximate Number of Plan Participants   4,000  
Defined Benefit Plan, Contributions by Employer   $ 6.7 0.2
Actuarial (gain) loss from remeasurement, net   $ (9.2) $ (30.2)
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate   3.90% 4.20%
Defined Benefit Plan, Expected Return on Plan Assets   $ 0.3 $ 0.8