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Employee Benefit Plans (Company's Benefit Obligations and Funded Status) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Change in projected benefit obligation:      
Service costs $ 0.1 $ 0.1 $ 0.2
Interest costs 9.6 10.2 6.0
Unfunded Supplemental Benefit Plans      
Change in projected benefit obligation:      
Benefit obligation at beginning of year 196.0 196.4  
Service costs 0.1 0.1  
Interest costs 9.6 10.2  
Actuarial (gain) loss (5.3) 4.8  
Benefits paid (15.3) (15.5)  
Projected benefit obligation at end of year 185.1 196.0 $ 196.4
Change in plan assets:      
Contributions 15.3 15.5  
Benefits paid (15.3) (15.5)  
Fair value of plan assets at end of year 0.0 0.0  
Reconciliation of funded status, Unfunded status of the plans 185.1 196.0  
Amounts recognized in the consolidated balance sheet, Accrued benefit liability 185.1 196.0  
Unrecognized net actuarial loss 43.3 50.9  
Amounts recognized in accumulated other comprehensive income/loss 43.3 50.9  
Accumulated benefit obligation at end of year $ 185.1 $ 196.0