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				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 29.3		$ 10.2		$ 44.7		$ 16.9
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	21.1		2.8		17.3		(6.8)
Interest Rate Swaps and Forwards | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	16.6		12.8		34.9		27.4
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	9.6	[1]	9.0	[1]	18.7	[1]	17.4	[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)	2.0	[2]	0.6	[3]	2.1	[3]	1.8	[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)	(0.4)	[4]	(0.4)	[4]	(0.8)	[4]	(0.8)	[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)	(0.6)	[1]	(0.3)	[1]	(0.8)	[1]	(0.6)	[1]
Foreign Exchange Contracts | Net Realized Investment Gain (Loss)
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	12.7		(2.6)		9.8		(10.5)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion)	$ 10.5	[2]	$ (6.1)	[3]	$ (1.9)	[3]	$ (24.6)	[3]

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

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

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

[4] Loss recognized in interest and debt expense