Gains and Losses on DerivativesNot Designated as Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Realized Investment Gain (Loss)
Derivative, Gain (Loss) on Derivative, Net	$ 11.0	$ (1.9)	$ 0
Not Designated as Hedging Instrument
Net Realized Investment Gain (Loss)
Derivative, Gain (Loss) on Derivative, Net	1.2	28.8	51.8
Not Designated as Hedging Instrument | Credit Default Swaps
Net Realized Investment Gain (Loss)
Derivative, Gain (Loss) on Derivative, Net	(0.3)	(1.9)	0
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Net Realized Investment Gain (Loss)
Derivative, Gain (Loss) on Derivative, Net	(1.8)	0	0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Net Realized Investment Gain (Loss)
Derivative, Gain (Loss) on Derivative, Net	$ 3.3	$ 30.7	$ 51.8