Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax [Abstract]
Income Tax Expense	$ 371.2	$ 139.9	$ 373.0
Additional Paid in Capital
Stock-Based Compensation	(3.6)	(3.0)	(0.8)
Other Comprehensive Income (Loss), Tax
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	(892.5)	725.8	(1,102.8)
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance, Tax Expense	856.6	(665.1)	743.3
Change in Net Gain on Cash Flow Hedges	(4.3)	(2.0)	(1.3)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax	(0.1)	0.0	0.0
Change in Unrecognized Pension and Postretirement Benefit Costs	3.2	(92.4)	185.2
Total	$ 330.5	$ 103.2	$ 196.6