v3.26.1
Accumulated Other Comprehensive Income (Loss) (AOCI) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Balance $ 5,512.2 $ 5,040.9 $ 5,524.3 $ 4,927.0
Accumulated other comprehensive income (loss), net of tax, Beginning Balance     (256.7)  
Change in unrealized losses on debt securities     (97.3)  
Change in foreign currency translation adjustment     (19.7)  
Amortization of net actuarial loss     1.0  
Tax effect 7.5 (7.2) 24.7 (39.4)
Accumulated other comprehensive income (loss), net of tax, Ending Balance (348.0)   (348.0)  
Balance 5,643.8 5,149.3 5,643.8 5,149.3
Unrealized Gains (Losses) on Debt Securities        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Balance     (147.6)  
Change in unrealized losses on debt securities     (97.3)  
Change in foreign currency translation adjustment     0.0  
Amortization of net actuarial loss     0.0  
Tax effect 7.6 (6.3) 24.7 (38.4)
Balance (220.2)   (220.2)  
Foreign Currency Translation Adjustment        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Balance     (76.3)  
Change in unrealized losses on debt securities     0.0  
Change in foreign currency translation adjustment     (19.7)  
Amortization of net actuarial loss     0.0  
Tax effect 0.1 (0.8) 0.3 (0.8)
Balance (95.7)   (95.7)  
Pension Benefit Adjustment        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Balance     (32.8)  
Change in unrealized losses on debt securities     0.0  
Change in foreign currency translation adjustment     0.0  
Amortization of net actuarial loss     1.0  
Tax effect     (0.3)  
Balance (32.1)   (32.1)  
Accumulated Other Comprehensive Income (Loss)        
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Balance (322.5) (399.8) (256.7) (496.4)
Balance $ (348.0) $ (345.6) $ (348.0) $ (345.6)