Derivative Financial Instruments - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Derivative [Line Items]
Current credit exposure on derivatives	$ 10,500,000		$ 10,500,000
Cash collateral from counterparties	34,500,000		34,500,000		45,600,000
Carrying value of fixed maturity securities posted as collateral to our counterparties	113,900,000		113,900,000		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	165,700,000		165,700,000		173,700,000
Notional Amount of forward starting interest rate swaps	245,000,000		245,000,000		335,000,000
Notional amount of foreign currency cash flow hedges	540,500,000		540,500,000		554,000,000
Notional Amount Of Derivatives Terminations	75,600,000	89,200,000	159,500,000	175,100,000
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	29,300,000	10,200,000	44,700,000	16,900,000
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	38,000,000
Forward Contracts [Member]
Derivative [Line Items]
Notional Amount Of Derivatives Terminations	21,000,000	12,000,000	56,000,000	31,900,000
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	500,000		600,000	100,000
Receive Variable/Pay Fixed | Swaps
Derivative [Line Items]
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	1,900,000	3,700,000	200,000	1,100,000
Receive Fixed/Pay Variable | Swaps
Derivative [Line Items]
Notional Amount Of Derivatives Terminations	45,000,000	50,000,000	90,000,000	105,000,000
Hedged benchmark interest rate, on the related interest rate swaps	4,200,000	9,300,000	4,200,000	7,200,000
Notional Amount of receive fixed, pay variable interest rate swap	$ 350,000,000		$ 350,000,000		$ 350,000,000