Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	$ 35.7	$ 11.0	$ (1.9)
Not Designated as Hedging Instrument
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	(37.9)	1.2	28.8
Not Designated as Hedging Instrument | Credit Default Swaps
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	0.1	(0.3)	(1.9)
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	(0.3)	(1.8)	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	$ (37.7)	$ 3.3	$ 30.7