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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Gain (Loss) on Investments [Line Items]
Gross Gains on Sales	$ 4.6	$ 14.8	$ 14.0	$ 33.0
Gross Losses on Sales	(2.3)	(5.4)	(9.0)	(22.1)
Other-Than-Temporary Impairment Loss	0	(1.2)	0	(4.2)
Gross Gains on Sales	2.3	1.6	2.3	7.1
Gross Losses on Sales	0	0	0	(0.2)
Impairment Loss	(1.3)	(0.4)	(1.8)	(0.4)
Embedded Derivative in Modified Coinsurance Arrangement	19.7	(33.7)	28.7	(24.4)
Foreign Currency Transactions	(1.7)	0.4	(2.6)	(1.1)
Net Realized Investment Gain (Loss)	$ 21.3	$ (23.9)	$ 31.6	$ (12.3)