Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss				$ (1.7)
Unrealized Gains (Losses) on Debt Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on sales of debt securities		$ (326.6)	$ (5.1)	(331.0)	$ (12.4)
Tax effect		84.6	1.2	85.7	3.1
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss	[1]	(0.6)	(0.4)	(1.7)	(1.5)
Tax effect	[1]	$ 0.1	$ 0.0	$ 0.4	$ 0.4

[1] Amounts are components of net periodic cost. See Note 12 Employee Benefit Plans for additional details.