Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Tax [Abstract]
Deferred Tax Asset, Reserves	$ 1,154.6	$ 668.5
Deferred Tax Asset, Employee Benefits	201.6	179.5
Deferred Tax Asset, Other	19.4	17.7
Deferred Tax Assets, Gross, Total	1,375.6	865.7
Less: Valuation Allowance	28.3	18.4
Deferred Tax Assets, Net of Valuation Allowance, Total	1,347.3	847.3
Deferred Tax Liabilities for Deferred Acquisition Cost	115.6	129.7
Deferred Tax Liabilities for Fixed Assets	58.5	56.2
Deferred Tax Liabilities on Invested Assets	1,213.7	493.4
Deferred Tax Liabilities, Intangible Assets	21.4	24.0
Deferred Tax Liability, Other	33.5	34.1
Gross Deferred Tax Liability	1,442.7	737.4
Deferred Tax Assets, Net	0.0	(109.9)
Deferred Tax Liabilities, Net, Total	$ 95.4	$ 0.0