Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Derivative
Credit Exposure on Derivatives			2.8
Cash Collateral from Counterparties	$ 34.3		$ 34.3				$ 15.4
Carrying Value of Fixed Maturity Securities Posted as Collateral to Counterparties	33.2		33.2				67.0
Cash Collateral to Counterparties	0.0		0.0				0.0
Aggregate Fair Value of all Derivative Instruments with Credit Risk-related Contingent Features in a Liability Position	56.5		56.5				92.9
Notional Amount of Derivatives	1,700.2	$ 1,701.6	1,700.2	$ 1,701.6		$ 1,638.7	1,687.4	$ 1,455.3	$ 1,477.4
Material Ineffectiveness on Cash Flow Hedges	0.0	0.0	0.0	0.0
Gain from Components Excluded from Assessment of Cash Flow Hedge Effectiveness	0.0	0.0	0.0	0.0
Approximate Amount of Net Deferred Gains on Derivative Instruments Expected to be Amortized During the Next Twelve Months			51.5
Material Ineffectiveness on Fair Value Hedges		0.0	0.0	0.0
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness		0.0	0.0	0.0
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge			0.0
Novated Foreign Currency Interest Rate Swap
Derivative
Notional Amount of Derivatives		97.0		97.0
Derivative Liability, Fair Value, Gross Liability		(29.5)		(29.5)
Foreign Exchange Contracts
Derivative
Hedge Gain Reclassification from AOCI to Earnings				13.1
Receive Variable/Pay Fixed | Swaps
Derivative
Notional Amount of Derivatives	150.0		150.0				150.0
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	0.7	2.3	2.8	4.4
Receive Fixed/Pay Variable | Swaps
Derivative
Notional Amount of Derivatives	600.0		600.0				600.0
Receive Fixed/Pay Variable | Interest Rate Swaps
Derivative
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(4.7)	3.6	(5.1)	(1.8)
Credit Default Swaps
Derivative
Notional Amount of Derivatives	70.0		70.0				97.0
Cash Flow Hedging | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	590.8		590.8				618.0
Long-term Debt
Derivative
Hedge Gain Reclassification from AOCI to Earnings				13.1
Long-term Debt | Subsequent Event
Derivative
Hedge Gain Reclassification from AOCI to Earnings					$ 27.6
Designated as Hedging Instrument | Foreign Currency Swap
Derivative
Notional Amount of Derivatives		124.7		124.7
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative
Notional Amount of Derivatives	$ 222.4		$ 222.4				$ 222.4
Not Designated as Hedging Instrument | Foreign Currency Swap
Derivative
Notional Amount of Derivatives		$ 125.4		$ 125.4