Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss				$ (1.1)
Unrealized Gains (Losses) on Debt Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on sales of debt securities		$ (1.0)	$ (2.8)	(4.4)	$ (7.3)
Tax effect		0.3	0.8	1.1	1.9
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss	[1]	(0.5)	(0.6)	(1.1)	(1.1)
Tax effect	[1]	$ 0.1	$ 0.2	$ 0.3	$ 0.4

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