Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Abstract]
Deferred Acquisition Cost	$ 118.6	$ 174.9
Deferred Tax Liabilities for Fixed Assets	52.8	80.7
Deferred Tax Liabilities on Invested Assets	1,074.0	1,427.6
Other	42.3	63.2
Gross Deferred Tax Liability	1,287.7	1,746.4
Reserves	908.0	1,308.5
Employee Benefits	188.7	307.4
Other	12.3	17.4
Gross Deferred Tax Asset	1,109.0	1,633.3
Less: Valuation Allowance	20.3	17.2
Net Deferred Tax Asset	1,088.7	1,616.1
Net Deferred Tax Liability	$ 199.0	$ 130.3