Income Tax (Benefit)(Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net Income	$ 355.1	$ 49.1	$ 441.2
Stock-Based Compensation	3.5	(3.3)	(2.7)
Other Comprehensive Income Investment Gain Tax	467.7	799.4	519.1
Change in Net Unrealized Gains and Losses on Securities Other-Than-Temporary Impaired Tax Expense	0	(1.1)	(0.5)
Change in Net Gain on Cash Flow Hedges	(4.3)	25.2	(5.0)
Change in Adjustment to Reserves for Future Policy and Contract Benefits Net of Reinsurance and Other tax expense benefit	(325.6)	(701.5)	(499.6)
Change in Foreign Currency Translation Adjustment	0	0	0.6
Change in Unrecognized Pension and Postretirement Benefit Costs	(68.0)	(67.4)	(12.7)
Total	$ 428.4	$ 100.4	$ 440.4