Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fixed Maturity Securities
Gross Gains on Sales									$ 74.0	$ 61.1	$ 48.6
Gross Losses on Sales									(24.0)	(41.3)	(83.5)
Other-Than-Temporary Impairment Loss									(19.9)	(15.9)	(211.8)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales									7.1	7.9	10.0
Gross Losses on Sales									(0.5)	(0.5)	(0.4)
Impairment Loss									(0.6)	(3.8)	(8.1)
Embedded Derivative in Modified Coinsurance Arrangement									(39.4)	21.1	243.1
Other Derivatives									0	0	12.3
Foreign Currency Transactions									(1.6)	(3.9)	1.5
Net Realized Investment Gain (Loss)	$ 7.4	$ (23.9)	$ (3.6)	$ 15.2	$ 27.5	$ 1.1	$ (29.5)	$ 25.6	$ (4.9)	$ 24.7	$ 11.7