Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fixed Maturity Securities
Gross Gains on Sales									$ 23.8	$ 9.3	$ 15.8
Gross Losses on Sales									(25.6)	(7.5)	(45.7)
Other-Than-Temporary Impairment Loss									(32.4)	(13.5)	(0.8)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales									16.0	21.2	15.6
Gross Losses on Sales									(0.1)	(0.8)	0.0
Impairment Loss									(5.9)	(3.4)	(2.0)
Gain (Loss) on Embedded Derivative									(37.7)	3.3	30.7
All Other Derivatives									35.7	11.0	(1.9)
Foreign Currency Transactions									(17.6)	(3.5)	(4.9)
Net Realized Investment Gain (Loss)	$ (2.7)	$ (26.6)	$ 0.8	$ (15.3)	$ (17.3)	$ 1.2	$ 25.9	$ 6.3	$ (43.8)	$ 16.1	$ 6.8