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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Credit) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Curtailment loss (gain) $ 0.5 $ 9.8 $ 0.0
Settlement charge 0.5 9.8 0.0
Pension Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost 2.0 1.5 2.6
Interest cost 21.4 28.0 27.3
Expected asset return (38.4) (41.1) (45.8)
Amortized actuarial loss 8.6 6.4 7.8
Amortized prior service cost (credit) 0.1 0.0 0.1
Curtailment loss (gain) 0.0 0.0 3.2
Settlement charge 0.5 10.4 0.4
Net periodic benefit cost (credit) (5.8) 5.2 (4.4)
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Service cost 0.4 0.3 0.4
Interest cost 10.2 12.8 12.4
Expected asset return 0.0 0.0 0.0
Amortized actuarial loss 1.0 0.1 0.8
Amortized prior service cost (credit) (1.5) (1.5) (2.7)
Curtailment loss (gain) 0.0 0.0 (0.6)
Settlement charge 0.0 0.0 0.0
Other postretirement benefit expense $ 10.1 $ 11.7 $ 10.3