Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss		$ (0.6)
Unrealized Gains (Losses) on Debt Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on sales of debt securities		(3.4)	$ (4.5)
Tax effect		0.8	1.1
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss	[1]	(0.6)	(0.5)
Tax effect	[1]	$ 0.2	$ 0.2

[1] Amounts are components of net periodic cost. See Note 11 Employee Benefit Plans for additional details.