Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Income Tax [Abstract]
Deferred Tax Assets, Investments	$ 354.5	$ 664.9
Deferred Tax Asset, Reserves	462.6	116.4
Deferred Tax Asset, Employee Benefits	161.9	158.3
Deferred Tax Asset, Other	35.8	34.4
Deferred Tax Assets, Gross, Total	1,014.8	974.0
Less: Valuation Allowance	11.0	10.3
Deferred Tax Assets, Net of Valuation Allowance, Total	1,003.8	963.7
Deferred Tax Liabilities for Deferred Acquisition Cost	175.1	177.7
Deferred Tax Liabilities for Fixed Assets	36.7	48.9
Deferred Tax Liabilities on Cost of Reinsurance	119.7	131.6
Deferred Tax Liability, Other	49.9	44.7
Gross Deferred Tax Liability	381.4	402.9
Deferred Income Tax	$ 622.4	$ 560.8