Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Fixed Maturity Securities
Gross Gains on Sales	$ 1.0	$ 2.6	$ 1.9	$ 9.2
Gross Losses on Sales	(1.1)	(1.6)	(6.5)	(7.1)
Other-Than-Temporary Impairment Loss	0	0	0	(0.8)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	2.4	1.4	9.8	9.0
Gross Losses on Sales	0	0	(0.6)	0
Impairment Loss	(0.4)	0	(3.4)	(1.0)
Gain on Embedded Derivative	12.4	12.4	20.9	18.8
Gain (Loss) on All Other Derivatives	12.7	(0.2)	12.3	(0.5)
Foreign Currency Transactions	(1.1)	(1.3)	(2.2)	(4.0)
Net Realized Investment Gain	$ 25.9	$ 13.3	$ 32.2	$ 23.6