Schedule of deferred income tax assets and liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred Acquisition Cost	$ 62.8	$ 35.6
Unrealized Gains and Losses	643.9	512.7
Other	252.0	138.2
Gross Deferred Tax Liability	958.7	686.5
Invested Assets	373.5	349.8
Employee Benefits	315.2	262.3
Other	0.6	29.7
Gross Deferred Tax Asset	689.3	641.8
Total Net Deferred Tax Liability	$ 269.4	$ 44.7