Derivative Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative [Line Items]
Credit Exposure on Derivatives	$ 5.5
Cash collateral from counterparties	(1.1)	0
Carrying value of fixed maturity securities posted as collateral to our counterparties	95.6	108.6
Cash Collateral to counterparties	0	(1.8)
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	135.6	170.5
Derivative, Notional Amount	1,477.4	1,432.8	1,413.0	1,681.9
Additions	291.0	336.0	46.9
Material Ineffectiveness on Cash Flow Hedges	0	0	0
Gain from Components Excluded from Assessment of Cash Flow Hedge Effectiveness	0	0	0
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months	46.7
Material Ineffectiveness on Fair Value Hedges	0	0	0
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	0	0	0
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as FV Hedge	0
Interest Rate Swaps | Cash Flow Hedge
Derivative [Line Items]
Derivative, Notional Amount	630.4	508.8
Foreign Currency Swap | Cash Flow Hedge
Derivative [Line Items]
Additions	150.0
Receive Variable/Pay Fixed | Cash Flow Hedge
Derivative [Line Items]
Derivative, Notional Amount		150.0
Receive Variable/Pay Fixed | Interest Rate Swaps
Derivative [Line Items]
Derivative, Notional Amount	150.0	174.0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	(11.5)	(1.2)	8.1
Receive Fixed/Pay Variable | Interest Rate Swaps
Derivative [Line Items]
Derivative, Notional Amount	600.0	600.0
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	21.1	(6.6)	(23.2)
Receive Fixed/Pay Variable | Fair Value Hedging [Member]
Derivative [Line Items]
Derivative, Notional Amount	600.0
Credit Default Swaps
Derivative [Line Items]
Derivative, Notional Amount	$ 97.0	$ 0