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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 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 gains (losses) on debt securities			41.0
Change in foreign currency translation adjustment			14.1
Amortization of net actuarial loss			1.1
Tax effect	$ 25.3	$ 88.8	(11.3)	$ 213.1
Accumulated other comprehensive income (loss), net of tax, Ending Balance	(824.0)		(824.0)
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 gains (losses) on debt securities			41.0
Tax effect	25.8	88.6	(10.6)	213.4
Accumulated other comprehensive income (loss), debt securities, available-for-sale, adjustment, after tax, Ending Balance	(721.2)		(721.2)
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			14.1
Tax effect	(0.3)	$ 0.6	(0.3)	$ 0.6
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax, Ending Balance	(68.3)		(68.3)
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			1.1
Tax effect			(0.4)
Accumulated other comprehensive (income) Loss, defined benefit plan, after Tax, Ending Balance	$ (34.5)		$ (34.5)