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Employee Benefit Plans (Company's Benefit Obligations and Funded Status) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Change in projected benefit obligation:      
Service costs $ 0.1 $ 0.2 $ 0.2
Interest costs 10.2 6.0 4.9
Unfunded Supplemental Benefit Plans      
Change in projected benefit obligation:      
Benefit obligation at beginning of year 196.4 262.1  
Service costs 0.1 0.2  
Interest costs 10.2 6.0  
Actuarial gains 4.8 (57.4)  
Benefits paid (15.5) (14.5)  
Projected benefit obligation at end of year 196.0 196.4 $ 262.1
Change in plan assets:      
Contributions 15.5 14.5  
Benefits paid (15.5) (14.5)  
Fair value of plan assets at end of year 0.0 0.0  
Reconciliation of funded status, Unfunded status of the plans 196.0 196.4  
Amounts recognized in the consolidated balance sheet, Accrued benefit liability 196.0 196.4  
Unrecognized net actuarial loss 50.9 48.1  
Amounts recognized in accumulated other comprehensive income/loss 50.9 48.1  
Accumulated benefit obligation at end of year $ 196.0 $ 196.4