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Accumulated Other Comprehensive Income (Loss) (AOCI) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]      
Accumulated other comprehensive income (loss), net of tax, Beginning Balance $ (655.8)    
Tax effect (73.8) $ (64.1) $ 248.3
Accumulated other comprehensive income (loss), net of tax, Ending Balance (496.4) (655.8)  
Unrealized Gains (Losses) on Debt Securities      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]      
Accumulated other comprehensive income (loss), debt securities, available-for-sale, adjustment, after tax, Beginning Balance (553.6) (751.6) 29.3
Change in unrealized gains (losses) on debt securities 272.7 262.3 (1,044.9)
Tax effect (72.8) (64.3) 264.0
Accumulated other comprehensive income (loss), debt securities, available-for-sale, adjustment, after tax, Ending Balance (353.7) (553.6) (751.6)
Foreign Currency Translation Adjustment      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]      
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax, Beginning Balance (64.9) (82.1) (39.9)
Change in foreign currency translation adjustment (47.1) 17.7 (43.3)
Tax effect 1.0 (0.5) 1.1
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax, Ending Balance (111.0) (64.9) (82.1)
Pension Benefit Adjustment      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]      
Accumulated other comprehensive (income) Loss, defined benefit plan, after Tax, Beginning Balance (37.3) (35.2) (81.8)
Net actuarial (loss) gain 5.3 (4.8) 57.4
Amortization of net actuarial loss 2.3 2.0 5.8
Amortization of prior service cost (credit)     0.2
Tax effect (2.0) 0.7 (16.8)
Accumulated other comprehensive (income) Loss, defined benefit plan, after Tax, Ending Balance (31.7) (37.3) (35.2)
Accumulated Other Comprehensive Income (Loss)      
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]      
Accumulated other comprehensive income (loss), net of tax, Beginning Balance (655.8) (868.9) (92.4)
Change in unrealized gains (losses) on debt securities 272.7 262.3 (1,044.9)
Change in foreign currency translation adjustment (47.1) 17.7 (43.3)
Net actuarial (loss) gain 5.3 (4.8) 57.4
Amortization of net actuarial loss 2.3 2.0 5.8
Amortization of prior service cost (credit)     0.2
Tax effect (73.8) (64.1) 248.3
Accumulated other comprehensive income (loss), net of tax, Ending Balance $ (496.4) $ (655.8) $ (868.9)