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, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	$ 5,040.9	$ 4,805.9	$ 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			149.7
Change in foreign currency translation adjustment			39.6
Amortization of net actuarial loss			0.9
Tax effect	(7.2)	1.7	(39.4)	18.2
Accumulated other comprehensive income (loss), net of tax, Ending Balance	(345.6)		(345.6)
Balance	5,149.3	4,832.6	5,149.3	4,832.6
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			149.7
Change in foreign currency translation adjustment			0.0
Amortization of net actuarial loss			0.0
Tax effect	(6.3)	1.6	(38.4)	17.9
Balance	(242.4)		(242.4)
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			39.6
Amortization of net actuarial loss			0.0
Tax effect	(0.8)	0.2	(0.8)	0.6
Balance	(72.2)		(72.2)
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			0.9
Tax effect			(0.2)
Balance	(31.0)		(31.0)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance	(399.8)	(720.5)	(496.4)	(655.8)
Balance	$ (345.6)	$ (729.7)	$ (345.6)	$ (729.7)