Income Tax (Benefit)(Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax [Abstract]
Income Tax Expense	$ 113.8	$ 347.1	$ 355.1
Additional Paid in Capital [Abstract]
Stock-Based Compensation	(3.0)	(0.8)	3.5
Other Comprehensive Income (Loss), Tax [Abstract]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	725.8	(1,102.8)	467.7
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance, Tax Expense	(665.1)	743.3	(325.6)
Change in Net Gain on Cash Flow Hedges	(2.0)	(1.3)	(4.3)
Change in Unrecognized Pension and Postretirement Benefit Costs	(92.4)	185.2	(68.0)
Total	$ 77.1	$ 170.7	$ 428.4