Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Abstract]
Deferred Acquisition Cost	$ 129.7	$ 118.6
Deferred Tax Liabilities for Fixed Assets	56.2	52.8
Deferred Tax Liabilities on Invested Assets	493.4	1,074.0
Deferred Tax Liabilities, Intangible Assets	24.0	6.5
Deferred Tax Liability, Other	34.1	35.8
Gross Deferred Tax Liability	737.4	1,287.7
Deferred Tax Asset, Reserves	668.5	908.0
Deferred Tax Asset, Employee Benefits	179.5	188.7
Deferred Tax Asset, Other	17.7	12.3
Gross Deferred Tax Asset	865.7	1,109.0
Less: Valuation Allowance	18.4	20.3
Net Deferred Tax Asset	847.3	1,088.7
Deferred Tax Assets, Net	(109.9)
Net Deferred Tax Liability	$ 0.0	$ 199.0