Derivative Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Current credit exposure on derivatives	$ 9,800,000		$ 9,800,000
Cash collateral from counterparties	15,300,000		15,300,000			45,600,000
Carrying value of fixed maturity securities posted as collateral to our counterparties	129,400,000		129,400,000			114,900,000
Cash Collateral to counterparties	0		0			0
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position	182,500,000		182,500,000			173,700,000
Notional Amount of forward starting interest rate swaps	200,000,000		200,000,000			335,000,000
Notional amount of foreign currency cash flow hedges	514,200,000		514,200,000			554,000,000
Notional Amount of Derivatives	1,513,200,000	1,498,700,000	1,513,200,000	1,498,700,000	1,309,500,000	1,413,000,000	1,538,700,000	1,681,900,000
Notional Amount Of Derivatives Terminations	71,300,000	55,000,000	230,800,000	230,100,000
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	5,500,000	59,400,000	50,200,000	76,300,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	0	0	0
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months			39,800,000
Forward Contracts [Member]
Derivative [Line Items]
Notional Amount of Derivatives	25,000,000	15,000,000	25,000,000	15,000,000	0	0	0	0
Notional Amount Of Derivatives Terminations	0	0	56,000,000	31,900,000
Receive Variable/Pay Fixed | Swaps
Derivative [Line Items]
Notional Amount of Derivatives	174,000,000	174,000,000	174,000,000	174,000,000	174,000,000	174,000,000	174,000,000	174,000,000
Notional Amount Of Derivatives Terminations	0	0	0	0
Notional amount of receive variable, pay fixed interest rate swaps to hedge changes in fair value of certain fixed rate securities held	174,000,000		174,000,000			174,000,000
Hedged benchmark interest rate, on the related interest rate swaps	200,000	7,600,000	400,000	8,700,000
Receive Fixed/Pay Variable | Swaps
Derivative [Line Items]
Notional Amount of Derivatives	800,000,000	735,000,000	800,000,000	735,000,000	595,000,000	685,000,000	785,000,000	890,000,000
Notional Amount Of Derivatives Terminations	45,000,000	50,000,000	135,000,000	155,000,000
Hedged benchmark interest rate, on the related interest rate swaps	4,100,000	14,800,000	8,300,000	22,000,000
Notional Amount of receive fixed, pay variable interest rate swap	600,000,000		600,000,000			350,000,000
Notional Amount of Interest Rate Derivatives	$ 250,000,000		$ 250,000,000