Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 72.7		$ (4.1)		$ 44.5
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	45.7		10.7		(41.6)
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)					12.3
Interest Rate Swaps | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	50.3		28.1		87.7
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	34.8	[1]	29.5	[1]	24.9	[1]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)					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
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	3.5	[2]	7.3	[2]	8.1	[2]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)					0
Interest Rate Swaps | Other Income [Member]
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)			(0.4)		(0.1)
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
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.6)	[3]	(0.5)	[4]
Foreign Exchange Contracts | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	0		0		(2.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(1.1)	[1]	(1.9)	[1]	(2.8)	[1]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)					0
Foreign Exchange Contracts | Net Realized Investments Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	22.4		(32.2)		(83.1)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	10.1	[2]	(25.6)	[2]	(73.4)	[2]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)					(2.0)
Foreign Exchange Contracts | Interest and Debt Expenses
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)			0		42.0
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)			2.3	[3]	1.7	[3]
Gain (Loss) Recognized in Income on Derivatives (Ineffective Portion)					$ 14.3

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

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

[3] Loss recognized in interest and debt expense

[4] Loss recognized in other income