Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Income Tax [Abstract]
Deferred Tax Asset, Reserves	$ 889.7	$ 1,279.6
Deferred Tax Asset, Employee Benefits	176.6	218.7
Deferred Tax Asset, Other	57.5	52.9
Deferred Tax Assets, Gross, Total	1,123.8	1,551.2
Less: Valuation Allowance	12.7	14.5
Deferred Tax Assets, Net of Valuation Allowance, Total	1,111.1	1,536.7
Deferred Tax Liabilities for Deferred Acquisition Cost	134.9	185.5
Deferred Tax Liabilities for Fixed Assets	71.1	74.7
Deferred Tax Liabilities on Invested Assets	1,144.9	1,443.5
Deferred Tax Liabilities on Cost of Reinsurance	171.6	180.4
Deferred Tax Liability, Other	47.0	68.7
Gross Deferred Tax Liability	1,569.5	1,952.8
Deferred Tax Liabilities, Net, Total	$ 458.4	$ 416.1