Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Income Tax [Abstract]
Deferred Tax Assets, Investments	$ 379.3	$ 593.2
Deferred Tax Asset, Employee Benefits	132.3	146.3
Deferred Tax Assets, Tax Credit Carryforwards	101.2	0.0
Deferred Tax Asset, Other	53.2	36.8
Deferred Tax Assets, Gross, Total	666.0	776.3
Less: Valuation Allowance	11.5	12.1
Deferred Tax Assets, Net of Valuation Allowance, Total	654.5	764.2
Deferred Tax Liabilities, Deferred Acquisition Cost	217.3	167.0
Deferred Tax Liabilities for Fixed Assets	40.1	13.8
Deferred Tax Liability, Reserves	67.2	85.9
Deferred Tax Liabilities, Cost of Reinsurance	234.8	110.4
Deferred Tax Liability, Other	54.4	48.4
Gross Deferred Tax Liability	613.8	425.5
Deferred Income Tax	$ 40.7	$ 338.7