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Pension and Other Post-retirement Benefits Net Periodic Pension Costs and Weighted Average Assumptions (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Dec. 31, 2019
Dec. 31, 2019
Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 1.5 $ 1.5 $ 0.1
Interest cost 21.7 21.3 25.4
Expected return on plan assets   (27.1) (36.5)
Curtailment gain recognized (2)   0.0 (3.3)
Settlement gain recognized (2)   0.0 (0.3)
Net periodic pension costs   $ (4.3) $ (14.6)
Cash balance interest credit rate   3.29% 3.29%
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 1.5   $ 0.1
Interest cost $ 21.3   24.9
Curtailment gain recognized (2)     $ (3.3)
Discount rate   3.82% 3.16%