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Pension and Other Postretirement Benefit Plans - Obligation and Funded Status (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in Plan Assets [Roll Forward]      
Non-Current Liabilities $ (191.8) $ (210.9)  
Other Investments 53.1 55.6  
Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated Benefit Obligation 745.4 698.8  
Change in Benefit Obligation [Roll Forward]      
Obligation, Beginning of Year 743.3 709.8  
Service Cost 10.2 8.1 $ 10.1
Interest Cost 32.5 33.2 29.9
Actuarial Loss 44.8 12.4  
Benefits Paid (51.0) (44.5)  
Participant Contributions 13.4 24.3  
Obligation, End of Year 793.2 743.3 709.8
Change in Plan Assets [Roll Forward]      
Fair Value, Beginning of Year 557.5 521.3  
Actual Return on Plan Assets 91.6 48.8  
Employer Contribution [1] 30.1 31.9  
Benefits Paid (51.0) (44.5)  
Fair Value, End of Year 628.2 557.5 521.3
Funded Status, End of Year (165.0) (185.8)  
Current Liabilities (1.4) (1.4)  
Non-Current Liabilities (163.6) (184.4)  
Postretirement Health and Life [Member]      
Change in Benefit Obligation [Roll Forward]      
Obligation, Beginning of Year 173.4 160.2  
Service Cost 4.4 3.9 4.3
Interest Cost 7.7 7.4 7.2
Actuarial Loss 15.5 11.9  
Benefits Paid (12.2) (13.1)  
Participant Contributions 3.1 3.1  
Plan Amendments (1.8) 0.0  
Obligation, End of Year 190.1 173.4 160.2
Change in Plan Assets [Roll Forward]      
Fair Value, Beginning of Year 154.3 153.4  
Actual Return on Plan Assets 24.5 9.6  
Employer Contribution 1.3 1.3  
Participant Contributions 3.1 3.1  
Benefits Paid (12.2) (13.1)  
Fair Value, End of Year 171.0 154.3 $ 153.4
Funded Status, End of Year (19.1) (19.1)  
Non-Current Assets 3.0 1.4  
Current Liabilities (1.1) (1.1)  
Non-Current Liabilities (21.0) (19.4)  
Irrevocable Grantor Trust [Member] | Postretirement Health and Life [Member]      
Change in Plan Assets [Roll Forward]      
Other Investments $ 19.2 $ 17.6  
[1] Includes Participant Contributions noted above.