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Pension and other Postretirement Benefits - Narrative (Schedule of Net Benefit Costs) (Details) - USD ($)
$ in Millions
 3 Months Ended 4 Months Ended 5 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Apr. 30, 2021
Sep. 30, 2021
Sep. 30, 2020
Retirement Benefits [Abstract]          
Defined Benefit Plan, Service Cost $ 0.0 $ 0.7 $ 0.0 $ 0.0 $ 2.0
Defined Benefit Plan, Interest Cost 5.8 6.6 6.6 9.6 19.5
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (9.5) (9.6) (12.1) (15.7) (28.6)
Defined Benefit Plan, Amortization of Gain (Loss)     0.1 0.0 0.0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) $ (3.7) $ (2.3) $ (5.4) $ (6.1) $ (7.1)