Location of and Gain (Loss) on Derivative Financial Instruments Not Designated as Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net Realized Investment Gain (Loss)
Gain (Loss) on Embedded Derivative	$ (0.3)	$ 19.7	$ 18.5	$ 28.7
Net Realized Investments Gain Loss
Net Realized Investment Gain (Loss)
Credit Default Swap Loss	(1.0)	0	(1.5)	0
Gain (Loss) on Embedded Derivative	(0.3)	19.7	18.5	28.7
Gains (Losses) on Derivatives Not Designated as Hedging Instruments	$ (1.3)	$ 19.7	$ 17.0	$ 28.7