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Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Curtailment loss (gain) $ 0.0 $ 3.2 $ 0.0
Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost 2.6 3.6 2.9
Interest cost 27.3 28.9 28.8
Expected asset return (45.8) (44.0) (42.1)
Amortized actuarial loss 7.8 7.1 5.5
Amortized prior service credit 0.1 (0.1) (0.1)
Curtailment loss (gain) 3.2 0.0 0.0
Settlement charge 0.4 3.2 0.0
Net periodic benefit credit (4.4) (1.3) (5.0)
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost 0.4 0.3 0.3
Interest cost 12.4 13.3 14.0
Expected asset return 0.0 0.0 0.0
Amortized actuarial loss 0.8 0.6 0.5
Amortized prior service credit (2.7) (2.7) (2.7)
Curtailment loss (gain) (0.6) 0.0 0.0
Settlement charge 0.0 0.0 0.0
Other postretirement benefit expense $ 10.3 $ 11.5 $ 12.1