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Accumulated Other Comprehensive Income (Loss) (AOCI) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]    
Balance $ 4,927.0 $ 4,862.8
Accumulated other comprehensive income (loss), net of tax, Beginning Balance (496.4)  
Change in unrealized losses on debt securities 124.4  
Change in foreign currency translation adjustment 3.9  
Amortization of net actuarial loss 0.5  
Tax effect (32.2) 16.5
Accumulated other comprehensive income (loss), net of tax, Ending Balance (399.8)  
Balance 5,040.9 4,805.9
Unrealized Gains (Losses) on Debt Securities    
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]    
Balance (353.7)  
Change in unrealized losses on debt securities 124.4  
Change in foreign currency translation adjustment 0.0  
Amortization of net actuarial loss 0.0  
Tax effect (32.1) 16.3
Balance (261.4)  
Foreign Currency Translation Adjustment    
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]    
Balance (111.0)  
Change in unrealized losses on debt securities 0.0  
Change in foreign currency translation adjustment 3.9  
Amortization of net actuarial loss 0.0  
Tax effect 0.0 0.4
Balance (107.1)  
Pension Benefit Adjustment    
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]    
Balance (31.7)  
Change in unrealized losses on debt securities 0.0  
Change in foreign currency translation adjustment 0.0  
Amortization of net actuarial loss 0.5  
Tax effect (0.1)  
Balance (31.3)  
Accumulated Other Comprehensive Income (Loss)    
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]    
Balance (496.4) (655.8)
Balance $ (399.8) $ (720.5)