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Defined Benefit and Postretirement Benefits (Components Of Net Periodic Cost) (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Pension        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ (0.2) $ 1.7 $ 2.4 $ 4.3
Interest cost 7.8 6.9 26.7 17.4
Expected return on plan assets (9.8) (15.7) (34.2) (43.6)
Amortization of prior service cost (benefit) 0.5 0.0 0.5 (0.1)
Amortization of net loss (benefit) 3.9 3.3 11.5 12.9
Curtailment and Settlement loss from discontinued operations     1.9 0.0
Net periodic cost 2.2 (3.8) 8.8 (9.1)
Pension | Discontinued Operations        
Defined Benefit Plan Disclosure [Line Items]        
Net periodic cost     1.9  
Postretirement        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.0 0.0 0.0 0.1
Interest cost (0.2) 0.2 0.3 0.5
Expected return on plan assets 0.0 0.0 0.0 0.0
Amortization of prior service cost (benefit) 0.2 (0.3) (0.3) (0.8)
Amortization of net loss (benefit) 0.2 0.0 (0.2) 0.0
Curtailment and Settlement loss from discontinued operations     0.0 0.0
Net periodic cost $ 0.2 $ (0.1) (0.2) $ (0.2)
Postretirement | Discontinued Operations        
Defined Benefit Plan Disclosure [Line Items]        
Net periodic cost     $ 0.2