Schedule of deferred income tax assets and liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Abstract]
Deferred Acquisition Cost	$ 174.9	$ 140.4
Deferred Tax Liabilities for Fixed Assets	80.7	87.1
Deferred Tax Liabilities on Invested Assets	1,427.6	1,128.4
Other	63.2	55.4
Gross Deferred Tax Liability	1,746.4	1,411.3
Reserves	1,308.5	1,060.5
Employee Benefits	307.4	246.0
Other	17.4	14.3
Gross Deferred Tax Asset	1,633.3	1,320.8
Less: Valuation Allowance	17.2	1.3
Net Deferred Tax Asset	1,616.1	1,319.5
Net Deferred Tax Liability	$ 130.3	$ 91.8