Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fixed Maturity Securities
Gross Gains on Sales									$ 15.8	$ 29.3	$ 74.0
Gross Losses on Sales									(45.7)	(20.4)	(24.0)
Other-Than-Temporary Impairment Loss									(0.8)	0	(19.9)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales									15.6	5.0	7.1
Other Investments Gross Realized Losses									0	(4.3)	(0.5)
Impairment Loss									(2.0)	(1.9)	(0.6)
Gain (Loss) on Embedded Derivative									30.7	51.8	(39.4)
Derivative, Gain (Loss) on Derivative, Net									(1.9)	0	0
Foreign Currency Transactions									(4.9)	(3.3)	(1.6)
Net Realized Investment Gain (Loss)	$ 9.3	$ (26.1)	$ 13.3	$ 10.3	$ 24.6	$ 21.3	$ (2.1)	$ 12.4	$ 6.8	$ 56.2	$ (4.9)