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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fixed Maturity Securities
Gross Gains on Sales									$ 29.3	$ 74.0	$ 61.1
Gross Losses on Sales									(20.4)	(24.0)	(41.3)
Other-Than-Temporary Impairment Loss									0	(19.9)	(15.9)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales									5.0	7.1	7.9
Gross Losses on Sales									(4.3)	(0.5)	(0.5)
Impairment Loss									(1.9)	(0.6)	(3.8)
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net									51.8	(39.4)	21.1
Foreign Currency Transactions									(3.3)	(1.6)	(3.9)
Net Realized Investment Gain (Loss)	$ 24.6	$ 21.3	$ (2.1)	$ 12.4	$ 7.4	$ (23.9)	$ (3.6)	$ 15.2	$ 56.2	$ (4.9)	$ 24.7