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Fair value of credit risk derivative asset after deduction of credit risk derivative liability.

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The net gain (loss) relating to components of the gain (loss) on the cash flow hedging instrument excluded from the assessment of cash flow hedge effectiveness. Recognized in earnings.

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Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative
Credit Exposure on Derivatives	$ 8.5
Cash collateral from counterparties	36.4	$ 15.4
Carrying value of fixed maturity securities posted as collateral to our counterparties	27.3	67.0
Cash Collateral to counterparties	0.0	0.0
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	50.2	92.9
Notional Amount of Derivatives	1,470.3	1,687.4	$ 1,477.4	$ 1,432.8
Notional Amount Of Derivatives Terminations	313.1	108.1	246.4
Material Ineffectiveness on Cash Flow Hedges	0.0	0.0	0.0
Gain from Components Excluded from Assessment of Cash Flow Hedge Effectiveness	0.0	0.0	0.0
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months	$ 53.6
Maximum Length of Time Hedged in Cash Flow Hedge	23 years
Material Ineffectiveness on Fair Value Hedges	$ 0.0	0.0	0.0
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	0.0	0.0	0.0
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as FV Hedge	0.0
Novated Foreign Currency Interest Rate Swap
Derivative
Notional Amount of Derivatives		97.0
Gross Derivative Liability		(29.5)
Foreign Exchange Contracts
Derivative
Hedge Gain Reclassification from AOCI to Earnings	8.2	13.1
Receive Variable/Pay Fixed \| Swaps
Derivative
Notional Amount of Derivatives	150.0	150.0
Receive Variable/Pay Fixed \| Interest Rate Swaps
Derivative
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	5.3	5.3	11.5
Receive Fixed/Pay Variable \| Swaps
Derivative
Notional Amount of Derivatives	600.0	600.0
Receive Fixed/Pay Variable \| Interest Rate Swaps
Derivative
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	0.1	(5.5)	21.1
Credit Default Swaps
Derivative
Notional Amount of Derivatives	70.0	97.0
Cash Flow Hedging \| Interest Rate Swaps
Derivative
Notional Amount of Derivatives	427.9	618.0
Notional Amount Of Derivatives Terminations	150.0		150.0
Long-term Debt
Derivative
Hedge Gain Reclassification from AOCI to Earnings	27.7	13.1
Designated as Hedging Instrument \| Foreign Currency Swap
Derivative
Notional Amount of Derivatives		124.7
Not Designated as Hedging Instrument \| Interest Rate Swaps
Derivative
Notional Amount of Derivatives	222.4	222.4
Not Designated as Hedging Instrument \| Foreign Currency Swap
Derivative
Notional Amount of Derivatives		125.4
Credit Default Swaps \| Swaps
Derivative
Notional Amount of Derivatives	70.0	97.0	97.0	$ 0.0
Notional Amount Of Derivatives Terminations	$ 29.0	$ 0.0	$ 0.0