Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Abstract]
Income Tax Expense	$ 409.8	$ 416.3	$ 371.2
Additional Paid in Capital
Stock-Based Compensation	0.0	(0.3)	(3.6)
Other Comprehensive Income (Loss), Tax
Other Comprehensive Income (Loss), Unrealized Holding Gain on Securities Arising During Period, Tax	328.1	318.0	(892.5)
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance, Tax Expense	(245.0)	(200.6)	856.6
Change in Net Gain on Cash Flow Hedges	(24.4)	(25.4)	(4.3)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax	0.0	0.0	(0.1)
Change in Unrecognized Pension and Postretirement Benefit Costs	(8.3)	(34.2)	3.2
Total	$ 460.2	$ 473.8	$ 330.5