Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 59.4		$ (42.2)		$ 76.3		$ 12.9
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	55.2		(12.5)		48.4		19.2
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	0		0		0		0
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	8.0		(7.7)		35.4		16.8
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	8.6	[1]	6.9	[1]	26.0	[1]	22.5	[1]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	0		0		0		0
Interest Rate Swaps | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	0.7	[2]	1.6	[2]	2.5	[2]	4.2	[2]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	0		0		0		0
Interest Rate Swaps | Other Income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)							0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)							(0.1)	[3]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)							0
Interest Rate Swaps | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.4)	[4]	(0.1)	[3]	(1.2)	[4]	(0.1)	[4]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	0		0		0		0
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		0		0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(0.3)	[1]	(0.4)	[1]	(0.9)	[1]	(1.5)	[1]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	0		0		0		0
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	51.4		(34.5)		40.9		(3.9)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	46.6	[2]	(21.2)	[2]	22.0	[2]	(7.7)	[2]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)	0		0		0		0
Foreign Exchange Contracts | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)			0				0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			0.7	[4]			1.9	[4]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)			$ 0				$ 0

[1] (1)Gain (loss) recognized in net investment income

[2] (2)Gain recognized in net realized investment gain (loss)

[3] (4)Loss recognized in other income

[4] (3)Loss recognized in interest and debt expense Three Months Ended September 30, 2010 Loss Recognized in OCIon Derivatives(Effective Portion) Gain (Loss) Reclassified fromAccumulated OCI into Income(Effective Portion) Gain (Loss) Recognizedin Income on Derivatives(Ineffective Portion) (in millions of dollars)Interest Rate Swaps and Forwards$(7.7) $6.9(1) $—Interest Rate Swaps— 1.6(2) —Interest Rate Swaps— (0.1)(3) —Foreign Exchange Contracts— (0.4)(1) —Foreign Exchange Contracts(34.5) (21.2)(2) —Foreign Exchange Contracts— 0.7(3) —Total$(42.2) $(12.5) $—