Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Income Tax [Abstract]
Deferred Tax Assets, Investments	$ 664.9	$ 0.0
Deferred Tax Asset, Reserves	0.0	889.7
Deferred Tax Asset, Employee Benefits	158.3	176.6
Deferred Tax Asset, Other	34.4	57.5
Deferred Tax Assets, Gross, Total	857.6	1,123.8
Less: Valuation Allowance	10.3	12.7
Deferred Tax Assets, Net of Valuation Allowance, Total	847.3	1,111.1
Deferred Tax Liabilities for Deferred Acquisition Cost	113.1	134.9
Deferred Tax Liabilities for Fixed Assets	48.9	71.1
Deferred Tax Liabilities on Invested Assets	0.0	1,144.9
Deferred Tax Liabilities, Other Comprehensive Income	43.2	0.0
Deferred Tax Liabilities on Cost of Reinsurance	156.8	171.6
Deferred Tax Liability, Other	44.7	47.0
Gross Deferred Tax Liability	406.7	1,569.5
Deferred Income Tax	$ 440.6
Deferred Tax Liabilities, Net, Total		$ (458.4)