Schedule of deferred income tax assets and liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Deferred Acquisition Cost	$ 257.0	$ 328.4
Unrealized Gains and Losses	507.8	392.7
Other	138.2	200.6
Gross Deferred Tax Liability	903.0	921.7
Invested Assets	349.8	317.8
Employee Benefits	262.3	174.2
Other	29.7	16.6
Gross Deferred Tax Asset	641.8	508.6
Less Valuation Allowance	0	4.1
Net Deferred Tax Asset	641.8	504.5
Total Net Deferred Tax Liability	$ 261.2	$ 417.2