Derivative Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Credit Exposure on Derivatives	$ 0.4		$ 0.4			$ 8.7
Cash collateral from counterparties	1.3		1.3			0
Carrying value of fixed maturity securities posted as collateral to our counterparties	91.5		91.5			108.6
Cash Collateral to counterparties	0		0			1.8
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	142.9		142.9			170.5
Notional amount of forward starting interest rate swaps to hedge anticipated purchase of long-term bonds	35.0		35.0			150.0
Notional amount of foreign currency cash flow hedges	481.1		481.1			508.8
Gain on Cash Flow Hedge Ineffectiveness	0	0	0	0
Gain from Components Excluded from Assessment of Cash Flow Hedge Effectiveness	0	0	0	0
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			45.7
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	0	0	0	0
Derivative, Notional Amount	1,336.1	1,513.2	1,336.1	1,513.2	1,403.7	1,432.8	1,309.5	1,413.0
Options Held on U.S. Treasury Rates
Derivative [Line Items]
Derivative, Notional Amount	0	0	0	0	10.0	0	0	0
Fair Value Hedging [Member]
Derivative [Line Items]
Gain (Loss) on Fair Value Hedge Ineffectiveness	0	0	0	0
Gain (Loss) from Hedged Firm Commitment Not Qualifying as Fair Value Hedge, Net			0
Receive Variable/Pay Fixed | Swaps
Derivative [Line Items]
Notional Amount of Interest Rate Fair Value Hedge Derivatives	150.0		150.0			174.0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(2.9)	0.2	(9.7)	0.4
Derivative, Notional Amount	150.0	174.0	150.0	174.0	174.0	174.0	174.0	174.0
Receive Fixed/Pay Variable | Swaps
Derivative [Line Items]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(1.8)	(4.1)	17.3	(8.3)
Notional Amount of Fair Value Hedge Instruments	600.0		600.0			600.0
Derivative, Notional Amount	635.0	800.0	635.0	800.0	675.0	750.0	595.0	685.0
Credit Default Swaps
Derivative [Line Items]
Derivative, Notional Amount	70.0		70.0			0
Credit Default Swaps | Swaps
Derivative [Line Items]
Derivative, Notional Amount	$ 70.0	$ 0	$ 70.0	$ 0	$ 70.0	$ 0	$ 0	$ 0