Location of Gains and Losses on Derivative Financial Instruments Designated as Cash Flow Hedging Instruments (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 5.5		$ 59.4		$ 50.2		$ 76.3
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	(5.5)		55.2		11.8		48.4
Interest Rate Swaps and Forwards | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	20.2		8.0		55.1		35.4
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	11.7	[1]	8.6	[1]	30.4	[1]	26.0	[1]
Interest Rate Swaps and Forwards | Net Realized Investment 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)	0.1	[2]	0.7	[3]	2.2	[3]	2.5	[3]
Interest Rate Swaps and Forwards | 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)	(0.5)	[4]	(0.4)	[4]	(1.3)	[4]	(1.2)	[4]
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)	(0.1)	[1]	(0.3)	[1]	(0.9)	[1]	(0.9)	[1]
Foreign Exchange Contracts | Net Realized Investment Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	(14.7)		51.4		(4.9)		40.9
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ (16.7)	[2]	$ 46.6	[3]	$ (18.6)	[3]	$ 22.0	[3]

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

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

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

[4] Loss recognized in interest and debt expense