Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Fixed Maturity Securities
Gross Gains on Sales	$ 0.9	$ 6.6
Gross Losses on Sales	(5.4)	(5.5)
Other-Than-Temporary Impairment Loss	0	(0.8)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	7.4	7.6
Other Investments Gross Realized Losses	(0.6)	0
Impairment Loss	(3.0)	(1.0)
Gain on Embedded Derivative	8.5	6.4
Derivative, Gain (Loss) on Derivative, Net	(0.4)	(0.3)
Foreign Currency Transactions	(1.1)	(2.7)
Net Realized Investment Gain	$ 6.3	$ 10.3