Income Tax (Benefit)(Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net Income	$ 21.8	$ 445.2	$ 439.7
Stock-Based Compensation	(3.3)	(2.7)	1.5
Other Comprehensive Income Investment Gain Tax	799.4	519.1	1,454.9
Change in Net Unrealized Gains and Losses on Securities Other-Than Temporarily Impaired	(1.1)	(0.5)	1.6
Change in Net Gain on Cash Flow Hedges	25.2	(5.0)	(45.3)
Change in Adjustment to Reserves for Future Policy and Contract Benefits Net of Reinsurance and Other tax expense benefit	(703.3)	(501.0)	(816.6)
Change in Foreign Currency Translation Adjustment	0	0.6	0
Change in Unrecognized Pension and Postretirement Benefit Costs	(67.4)	(12.7)	42.0
Adoption of ASC 320 Update - Note 1	0	0	7.7
Total	$ 71.3	$ 443.0	$ 1,085.5