Accumulated Other Comprehensive Income (Loss) (AOCI) - Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 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	146.5
Change in foreign currency translation adjustment	2.6
Amortization of net actuarial loss	0.5
Tax effect	(36.6)	$ 124.3
Accumulated other comprehensive income (loss), net of tax, Ending Balance	(755.9)
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	146.5
Tax effect	(36.4)	124.8
Accumulated other comprehensive income (loss), debt securities, available-for-sale, adjustment, after tax, Ending Balance	(641.5)
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	2.6
Tax effect	0.0	$ 0.0
Accumulated other comprehensive income (loss), foreign currency translation adjustment, net of tax, Ending Balance	(79.5)
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	0.5
Tax effect	(0.2)
Accumulated other comprehensive (income) Loss, defined benefit plan, after Tax, Ending Balance	$ (34.9)