Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fixed Maturity Securities
Gross Gains on Sales	$ 4.3	$ 4.6	$ 13.5	$ 14.0
Gross Losses on Sales	(33.0)	(2.3)	(40.1)	(9.0)
Other-Than-Temporary Impairment Loss	0	0	(0.8)	0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	5.6	2.3	14.6	2.3
Impairment Loss	(1.0)	(1.3)	(2.0)	(1.8)
Gain (Loss) on Embedded Derivative	(0.3)	19.7	18.5	28.7
Derivative, Gain (Loss) on Derivative, Net	(1.0)	0	(1.5)	0
Foreign Currency Transactions	(0.7)	(1.7)	(4.7)	(2.6)
Net Realized Investment Gain (Loss)	$ (26.1)	$ 21.3	$ (2.5)	$ 31.6