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Accumulated Other Comprehensive Income (Loss) (AOCI) (Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]        
Accumulated other comprehensive income (loss), net of tax, Beginning Balance     $ (868.9)  
Change in unrealized losses on debt securities     (187.5)  
Change in foreign currency translation adjustment     (0.6)  
Amortization of net actuarial loss     1.5  
Tax effect $ 57.2 $ 91.3 45.9 $ 304.4
Accumulated other comprehensive income (loss), net of tax, Ending Balance (1,009.6)   (1,009.6)  
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     (751.6)  
Change in unrealized losses on debt securities     (187.5)  
Tax effect 56.9 90.8 46.3 304.2
Accumulated other comprehensive income (loss), debt securities, available-for-sale, adjustment, after tax, Ending Balance (892.8)   (892.8)  
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     (82.1)  
Change in foreign currency translation adjustment     (0.6)  
Tax effect 0.3 $ 0.8 0.0 $ 1.4
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax, Ending Balance (82.7)   (82.7)  
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     (35.2)  
Amortization of net actuarial loss     1.5  
Tax effect     (0.4)  
Accumulated other comprehensive (income) Loss, defined benefit plan, after Tax, Ending Balance $ (34.1)   $ (34.1)