Derivative Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative [Line Items]
Current credit exposure on derivatives	$ 8,500,000
Cash collateral from counterparties	0		0
Carrying value of fixed maturity securities posted as collateral to our counterparties	92,400,000		108,600,000
Cash Collateral to counterparties	1,800,000		1,800,000
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	157,400,000		170,500,000
Notional amount of forward starting interest rate swaps to hedge anticipated purchase of long-term bonds	110,000,000		150,000,000
Notional amount of foreign currency cash flow hedges	490,800,000		508,800,000
Gain on Cash Flow Hedge Ineffectiveness	0	0
Gain from Components Excluded from Assessment of Cash Flow Hedge Effectiveness	0	0
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months	43,200,000
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	0	0
Notional Amount Of Derivatives Additions	94,000,000	35,000,000
Fair Value Hedging [Member]
Derivative [Line Items]
Gain on Fair Value Hedge Ineffectiveness	0	0
Gain (Loss) from Hedged Firm Commitment Not Qualifying as Fair Value Hedge, Net	0	0
Receive Variable/Pay Fixed | Swaps
Derivative [Line Items]
Notional Amount of Interest Rate Fair Value Hedge Derivatives	174,000,000		174,000,000
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	2,400,000	1,700,000
Notional Amount Of Derivatives Additions	0	0
Receive Fixed/Pay Variable | Swaps
Derivative [Line Items]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	4,100,000
Notional Amount of Fair Value Hedge Instruments	600,000,000		600,000,000
Notional Amount Of Derivatives Additions	0	0
Credit Default Swap [Member]
Derivative [Line Items]
Notional Amount Of Derivatives Additions	70,000,000
Credit Default Swap [Member] | Swaps
Derivative [Line Items]
Notional Amount Of Derivatives Additions	$ 70,000,000	$ 0