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, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fixed Maturity Securities
Gross Gains on Sales	$ 3.5	$ 8.7	$ 9.4	$ 18.2
Gross Losses on Sales	(1.7)	(9.8)	(6.7)	(16.7)
Other-Than-Temporary Impairment Loss	0	(0.7)	0	(3.0)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0	4.3	0	5.5
Gross Losses on Sales	0	(0.2)	0	(0.2)
Impairment Loss	0	0	(0.5)	0
Embedded Derivative in Modified Coinsurance Arrangement	(3.3)	(4.8)	9.0	9.3
Foreign Currency Transactions	(0.6)	(1.1)	(0.9)	(1.5)
Net Realized Investment Gain (Loss)	$ (2.1)	$ (3.6)	$ 10.3	$ 11.6