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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Derivative
Credit Exposure on Derivatives	$ 0.8
Cash collateral from counterparties	25.4	$ 36.4
Carrying value of fixed maturity securities posted as collateral to our counterparties	35.2	27.3
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	52.8	50.2
Notional Amount of Derivatives	1,052.0	1,470.3	$ 1,687.4	$ 1,477.4
Notional Amount Of Derivatives Terminations	451.8	313.1	108.1
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	$ 58.0
Maximum Length of Time Hedged in Cash Flow Hedge	22 years
Loss on Contract Termination	$ 1.2
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
Derivative, Amount of Hedged Item	3.5
Foreign Exchange Contracts
Derivative
Hedge Gain Reclassification from AOCI to Earnings		8.2	13.1
Novated Foreign Currency Interest Rate Swap
Derivative
Notional Amount of Derivatives			97.0
Receive Variable/Pay Fixed
Derivative
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	5.9	5.3	5.3
Receive Fixed/Pay Variable
Derivative
Notional Amount of Derivatives	250.0	600.0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	2.8	0.1	(5.5)
Receive Variable/Pay Fixed \| Swaps
Derivative
Notional Amount of Derivatives	105.5	150.0	150.0	150.0
Notional Amount Of Derivatives Terminations	48.0	0.0	0.0
Credit Default Swaps \| Swaps
Derivative
Notional Amount of Derivatives	70.0	70.0	97.0	$ 97.0
Notional Amount Of Derivatives Terminations	0.0	29.0	0.0
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
Not Designated as Hedging Instrument \| Receive Variable/Pay Fixed
Derivative
Notional Amount of Derivatives	3.5
Not Designated as Hedging Instrument \| Credit Default Swaps
Derivative
Notional Amount of Derivatives	70.0	70.0
Long-term Debt
Derivative
Hedge Gain Reclassification from AOCI to Earnings		27.7	13.1
Cash Flow Hedging \| Interest Rate Swaps
Derivative
Notional Amount of Derivatives	394.1	427.9
Notional Amount Of Derivatives Terminations		150.0
Cash Flow Hedging \| Novated Foreign Currency Interest Rate Swap
Derivative
Gross Derivative Liability			(29.5)
Cash Flow Hedging \| Designated as Hedging Instrument \| Foreign Currency Swap
Derivative
Notional Amount of Derivatives			$ 124.7
Fair Value Hedging \| Receive Variable/Pay Fixed
Derivative
Notional Amount of Derivatives	102.0	150.0
Fair Value Hedging \| Receive Fixed/Pay Variable
Derivative
Notional Amount of Derivatives	250.0	$ 600.0
Notional Amount of Derivatives Matured	350.0
Amount of Hedged Item, Matured	350.0
Fair Value Hedging \| Receive Variable/Pay Fixed \| Swaps
Derivative
Notional Amount Of Derivatives Terminations	24.0
Net
Derivative
Cash collateral from counterparties	$ 26.1