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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss				$ (1.4)
Unrealized Gains (Losses) on Debt Securities
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net realized losses on sales of debt securities		$ (2.0)	$ (326.6)	(8.9)	$ (331.0)
Tax effect		0.4	84.6	2.2	85.7
Pension Benefit Adjustment
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of net actuarial loss	[1]	(0.5)	(0.6)	(1.4)	(1.7)
Tax effect	[1]	$ 0.2	$ 0.1	$ 0.4	$ 0.4
[1] Amounts are components of net periodic cost. See Note 11 Employee Benefit Plans for additional details.

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Amount before tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount, before tax, of reclassification adjustment from accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

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