Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	$ (0.1)	$ (1.5)	$ 35.7
Not Designated as Hedging Instrument
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	30.7	40.6	(37.9)
Not Designated as Hedging Instrument | Credit Default Swaps
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	(0.4)	(0.9)	0.1
Not Designated as Hedging Instrument | Interest Rate Swaps
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	(0.1)	0.4	0.0
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	0.4	0.2	(0.3)
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Net Realized Investment Gain (Loss)
Net Gain (Loss) on Derivatives	$ 30.8	$ 40.9	$ (37.7)