Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax [Abstract]
Deferred Tax Asset, Reserves	$ 1,279.6	$ 1,154.6
Deferred Tax Asset, Employee Benefits	218.7	201.6
Deferred Tax Asset, Other	52.9	19.4
Deferred Tax Assets, Gross, Total	1,551.2	1,375.6
Less: Valuation Allowance	14.5	28.3
Deferred Tax Assets, Net of Valuation Allowance, Total	1,536.7	1,347.3
Deferred Tax Liabilities for Deferred Acquisition Cost	185.5	115.6
Deferred Tax Liabilities for Fixed Assets	74.7	58.5
Deferred Tax Liabilities on Invested Assets	1,443.5	1,213.7
Deferred Tax Liabilities on Cost of Reinsurance	180.4	10.8
Deferred Tax Liability, Other	68.7	44.1
Gross Deferred Tax Liability	1,952.8	1,442.7
Deferred Tax Liabilities, Net, Total	$ 416.1	$ 95.4