Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Income Tax [Abstract]
Deferred Tax Assets, Investments	$ 593.2	$ 354.5
Deferred Tax Asset, Reserves	0.0	462.6
Deferred Tax Asset, Employee Benefits	146.3	161.9
Deferred Tax Asset, Other	36.8	35.8
Deferred Tax Assets, Gross, Total	776.3	1,014.8
Less: Valuation Allowance	12.1	11.0
Deferred Tax Assets, Net of Valuation Allowance, Total	764.2	1,003.8
Deferred Tax Liabilities, Deferred Acquisition Cost	167.0	175.1
Deferred Tax Liability, Reserves	85.9	0.0
Deferred Tax Liabilities, Cost of Reinsurance	110.4	119.7
Deferred Tax Liability, Other	62.2	86.6
Gross Deferred Tax Liability	425.5	381.4
Deferred Income Tax	$ 338.7	$ 622.4