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Employee Benefit Plans Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost $ 3.6 $ 2.9 $ 3.2
Interest cost 28.9 28.8 28.6
Expected asset return (44.0) (42.1) (42.0)
Amortized actuarial loss 7.1 5.5 6.0
Amortized prior service credit (0.1) (0.1) (0.1)
Settlement charge 3.2 0.0 0.0
Net periodic benefit credit (1.3) (5.0) (4.3)
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost 0.3 0.3 0.4
Interest cost 13.3 14.0 15.0
Expected asset return 0.0 0.0 0.0
Amortized actuarial loss 0.6 0.5 0.8
Amortized prior service credit (2.7) (2.7) (2.7)
Settlement charge 0.0 0.0 0.0
Other postretirement benefit expense $ 11.5 $ 12.1 $ 13.5